What new motherboard should I buy to replace the one in my old Gateway 700XL (Intel D850EMV2)?

A month or two ago, I believe my NVIDIA graphics card's fan died ( http://support.gateway.com/s//VIDCARD/VisionTEk/6002322/6002322nv.shtml ). As soon video artifacts caused by the overheating chip made it impossible to use the machine, it was decided that a new AGP graphics card should be obtained. The newly-purchased BFG GeForce 7800 ( http://www.bfgtech.com/7800GS_256.html ), however, required more power than our current power-supply could support.

I purchased a new BFG 650-watt power supply (http://www.bfgtech.com/power_supply_650W.html) to replace the old 250-watt supply in preparation for the graphics card upgrade. The new power supply didn't fit in Gateway's chassis ( http://support.gateway.com/s//CASES/3501242/3501242nv.shtml ), but some leather lacing took care of securing that into place. Unfortunately, after everything was connected, I realized that the new power supply was missing an Aux ATX plug ( http://support.gateway.com/s/MOTHERBD/INTEL/2515762/2515762mvr6.shtml ) that our motherboard required. I purchased an adapter that would create this plug on the new power supply ( http://www.pcpartscollection.com/20atxpoextca.html ).

So here we are, on an upgrade rampage. At this point, why not order a new processor to replace the old 2.8 GHz one? Suddenly, we've purchased a Pentium 4 Extreme Edition ( http://www.geeks.com/details.asp?invtid=P43400F478&cpc=SCH ). Uh-oh. Maybe that was a little too impulsive... I don't think my current motherboard will support that chip (motherboard information below). So now what? New motherboard? As it just so happens, these two boards seem to match my needs: http://www.intel.com/products/motherboard/D865PERL/index.htm   and   http://www.intel.com/products/motherboard/d865gbf/index.htm  .  I'm not entirely sure what makes these two boards different. BUT WAIT! I use RDRAM (information on the RAM below). I will need to purchase NEW, compatible RAM if I am to buy one of the two new motherboards (right?).

+++++My Questions+++++++++++++++

Below I have listed information regarding my current machine's components. What should I do? Which motherboard should I buy? Will I need new RAM? Should I be looking into a brand-new case at this point?

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My old Gateway 700XL uses an Intel D850EMV2 motherboard ( http://www.intel.com/design/motherbd/mv2/ ).

Additional Motherboard Information:

Manufacturer : Intel Corporation
MP Support : No
Model : D850EMV2
Version : AAA87987-401
System BIOS : 51-2300-000000-00101111-030199-

I'm using the following processor currently:

Processor
Model : Intel(R) Pentium(R) 4 CPU 2.80GHz
Speed : 2.78GHz
Performance Rating : PR3035 (estimated)
Cores per Processor : 1 Unit(s)
Threads per Core : 1 Unit(s)
Package : FC µPGA478
Rated Speed/FSB : 2800MHz / 4x 133MHz
Multiplier : 21/1x
Minimum/Maximum Multiplier : 0/1x / 21/1x
Generation : G8
Name : P4N (Northwood) Pentium 4C 130nm 1.6-3.4GHz 1.475-1.575V
Revision/Stepping : 2 / 7 (9)
Stepping Mask : C1
Microcode : MU0F2737
Core Voltage Rating : 1.550V
Maximum Physical / Virtual Addressing : 36-bit / 32-bit
Native Page Size : 4kB

There are four sticks of the following RAM, totaling 2GB:

Model : MR16R 162GAF0-CM8
Type : 512MB RDRAM RIMM
Technology : 16x(256Mx16)
Speed : PC800-40
Standard Timings : 2/2 (tCAC/tRCD)
Version : 1.0
Date of Manufacture : Tuesday, September 10, 2002
Set Timing @ 411MHz : 2/2 (tCAC/tRCD)

There are 4 IDE devices in the machine (two Western Digital ATA 120-GB 7200RPM Hard Drives R1 - WDC WD1200BB-53CAA1, one MATSHITA DVD-RAM LF-D311, one CDRW drive, and a floppy which seems inconsequential and thus is not included in the total number of IDE devices). 4 of the 5 PCI slots in the machine are in use. The AGP slot is home to the graphics card.