try to buy a powerful PC?

"What is most important aspect of the computer? SCSI hard drive , cpu clock, dual cpu capable, rdram"

Since changing video formats is a strictly linear process, multiple CPUs will not help at all.  Also, depending on the program used, it probably reads a little from disk, manipulates, stores to disk rather than read everything to RAM, manipulate, save to disk.  

So, I'd recommend the fastest possible CPU, like an Athlon 2000+ or P4 2Ghz with not too much RAM, like 256MB, and a pair of matching ATA-100 7200RPM IDE drives on a RAID controller set to 'mirror' or level 1.  (There is no need for SCSI in this application.  SCSI is not noticably faster in single user environments and costs a LOT more.)  

The system described can be had for well under $1000 if you put it together yourself.

A fast HD makes a big difference when playing with large media files. This is why film companies using FinalCut Pro invest half their budget in Ultra 160 Raid setups. Dual CPU can also help assuming youre using something like Media Cleaner which can take advantage of them. Ram is less important because the data isnt stored their long. It comes off the disk, processed, and dumped back to the disk very quickly.

Personally i'd try to pick up a used Mac G4. Apple has specialized in kicking media butt for a long time and you should be able to pick up a nice one for $1k.

I'd like to throw in my 2 Cents also.

I think that all the comments so far are very relevant. It depends on how deep you want to get into the video editing.
If you are using prodessional programs that support dual CPU's - then go for it !
If you will be doing heavy duty file and format changing, it may be best to go SCSI with 2 or 3 drives, preferably one for the source file and one for the destination file.
with the third for the OS.

Ram can be a big plus, so start with 256MB as a minimum, or 4 times the recommended RAM for the program you will be using.

I hope this helps !

Keep in mind that 2Gig P4 is 100 more expenive than P4 1.8 but will give you only 10% more processor power wich is usually unnoticable. Also if you deside to build your one P4 based computer, always by newer P4 "Nortwood" wiht 512K cash it is almost same price but faster.

Thanks for so many inputs.  Is the workstation line better than the desktop line?
Dell has dimension for destop  and precision for workstation.  The CPU
memory configuration seems to be similar, but dell.com says the precision
is top of line in performance.  I just don't understand what makes them special.

Below is a Dell Workstation finished in ebay for $730
CPU Intel Pentium? 4 Processor at 1.5GHz with 256KB Cache
 Memory: 512MB PC800 ECC RDRAM(2 RIMMs, support up to 2GB in 4 RIMMs)
Video: ATI Rage 128 Ultra 32MB  AGP Video Card
Modem/Network Card: Integrated 3Com? 10/100mbps 3C905C fast network controller with Wakeup on LAN
 Floppy Drive: 1.44MB Floppy Disk Drive
 Hard Disk Drive: 20 GB EIDE Hard Drive, 7200RPM

They are better in terms of quality of motherboards, for the most part. The mobo's on the precision are easier to upgrade in the future, and almost always need a sound card and video card where alot of there dimensions have these integrated into the mobo already. This is not to say that some dimensions do not have these because the pricier ones are pretty much identical to the workstation line of computers. However they try to gear the workstations more towards businesses and the dimensions for a typical consumer. The dimension is usually cheaper though, all else being equal.