Is there a cheaper alternative to Dell's OEM motherboard.

$100 for something similar to whats in there, I suppose.  If there is something better, more efficient/reliable, than I could see spending more money.

Is it worth buying a direct replacement, or getting a motherboard/cpu combo?

firstly most del's will only allow you to put a dell mobo in there, as the screws wont line up in the case
what spec was it?

I was looking at the pictures, and it appears that the reari/o panel and pci slots are upside down for the dimension e520 case. The mobo mounts to the left side of the machine, I don't thin this is standard.

As you face the front of it, is the mobo on the left? And are the expansion slots at the top or at the bottom?  If the mb is on the left and the slots are at the top it could still be a standard design, if the mb is on the left but the slots are on the bottom then it is a btx chassis and mb and you'd be best off buying a new case (and maybe power supply) to put the new mb in.

These Antec cases are fairly basic but even better is that it comes with an earthpower power supply.  Its an 80+ high efficiency power supply and the company has been around for ages cuz they have very good quality.

How much is Dell trying to charge you for a replacement system board, $300 or so?

Facing the front of the case MB is on left side, and slots are on the bottom, but the power supply is on top like atx. I think the best bet at this point is to get a new case, and MB.

I thought I was going crazy. Then I looked at other machines at my office, and realized my Dell is 'donkey' backwards.

I am going to pull the fan case to see what exactly I need.

If I have "Intel Pentium D Processor 925 with Dual Core Technology(3GHz,800FSB)" What MB will I need?
Can I use something other than 800FSB?

I'm pretty lost once I get this far into a PC.

Quick notes: its a lga 775. also will I need a new heatsink/fan with a new case/motherboard?

A couple of caveats r.e. replacing this with a non-Dell board:

=>  First, it's a BTX motherboard, NOT an ATX board -- so you'll need a BTX board to replace it with.

=>  Second, the front panel connection on most Dell's is non-standard, so you may need to jury-rig something to use a non-Dell board.

=>  Third, the Dell install media is likely a BiOS-tied version of XP that doesn't require activation because of the Dell BIOS.   This would not work with a non-Dell board (so plan on buying an OS if you don't have a copy you can use).

Bottom Line:   Unless you're willing to replace the case it's best to use a Dell replacement board.

... another note:  The E520 motherboard is often available on e-bay for ~ $150.   There are none at this moment (one listing for a factory-sealed board for $159 just expired today) ... but there are several prior sales in that price range.   If you DO want to just replace the board, I'd check there every few days ... a lot cheaper than a new replacement from Dell.

garycase's recommendation for buying a board from ebay sounds like a good cheap option and his third point is valid if you had to install from their media (oem media checks to verify it is being installed on their machine though I've only experienced that from HP so far, I've gotten Dell media to install on another machine the one time I tried it) but if the hard drive is good you might only have to run a reactivation, if even that, to get windows up and running with a new mb.

Your pentium D925 should work on all of the boards I've referenced but that processor architecture is a little long in the tooth.  When you look for a new cpu you'll have a lot of options with those boards as well.

The LGA775 socket is the same on all of those boards so your heat-sink/fan will be fine.

Pretty sure I am going to buy a new case at this point.  

Hmm... been thinking a bit more on this, does your hard drive and/or optical drives use SATA or PATA.  Most of the modern boards only have one channel (2 devices per channel)for PATA and you don't want to stick a hard drive and optical drive on the same channel as it will slow down your hard drive horribly.

If you aren't sure about the difference, PATA uses a 40 pin connector that is a little over 2 inches wide whereas SATA uses 7 pin connector that is a little over half an inch wide.

If both your optical and hard drive uses PATA then the boards I mentioned above won't work for you.  The newest mb I could find with 2 channels uses an nvidia 650i chipset which is advertised for accepting Penryn, the latest 45nm Intel CPUs(if you were to consider upgrading down the road) but here is the one I found: http://www.newegg.com/Product/Product.aspx?Item=N82E16813157120

Take note that this last board does not have integrated video, if your existing system doesn't have a discrete PCI Express video card then you'll need to purchase one.

A little search for info on your mb and I found this: http://support.dell.com/support/edocs/systems/dime520/en/SM_EN/techov.htm#wp1058256 which shows the mb with 4 SATA ports and only one PATA channel, so if your board is like the one depicted you should be fine with with only one PATA channel.  

This post has dragged out a bit, but let me know if you need more help.

Putting a hard drive and optical drive on the same channel does NOT slow down the hard drive transfers ... the only potential slowdown is when you're transferring between the optical drive and hard drive; and even then, that's not an issue, since the data can't move between them any faster than the optical drive's rate anyway.

But I agree you need to be sure you have enough I/O connections to use whatever current components you need to connect ... or buy an add-in card to add the additional I/O.

I have two sata drives, actually I think all is currently on Sata.
This has definately turned into a conversation, but Thank you!