I've pretty much used ATI based cards.
Same here until recently, I've used 9700pro, 9800xt/pro x800pro, x850pro, and my brothers HD series I think 3750? also used several versions of the Nvidia 8800gts's and found one that I liked, 8800gts 640mb factory OC, even though it's not the "new" core, and was a bit slower than at similar priced "faster new core" card, it's still pretty fast.. The new nvidia cards are sweet. I don't have a preference, other than one that works with the games that I like to play. Unless you are a GL or CAD only guy, ATI and Nvidia are just different flavors of the same icecream... It depends on what you want bundled with the card, how much money you wanna spend to get the high fps high res gameplay etc. I'm finding that SLI and Crossfire are not quite what they were "hyped up to be" they don't seem to work "all the time" and seem to be limited to "software that supports it"

I think one 9800GTX+ OC would work well with your current CPU, but would work even better if you upgraded to say, E8400 to E8600.. Whichever card you like, be sure that it has lots of memory, hopefully gddr3 or better in the range of 640MB to 1GB..

You can always get another 9800gtx and run SLI if you can afford to wait on the rebates.

Be sure to read up on how to handle mail in rebates... Sometimes they don't like to give you the money/loose your paperwork in the round file cabinet..

I'm hearing a lot of good stuff about NVidia. Got any suggestions there? Also, does the fact that you are suggesting a single card mean you think it's a better option than running 2 cards?
I'm not saying that one is better than two, but you need to have some serious CPU power and memory bandwidth to take advantage of a dual GPU/SLI/crossfire setup. That and This stuff isn't like voodoo SLI was, it doesn't just equal "faster better gaming" not that I can notice anyway, and I've been using a pair of 8800gts's in SLI for a while, also have been fooling with a pair of ATI HD3870's.. Those cards have major driver issues with different titles... each title may require a custom 3d setting/graphics feature "profile" before loading the EXE, just to get the game to display correctly... This might not be using the "wow" uber anti-alliasing high color, high resolution better gaming framerates that "sli" or "crossfire" benchmarks have them hyped up on.. I have to run some games with these AA features OFF, due to driver/software issues...

If you have a p4 dual core with HTT. Newer core2 8400 to 8600 CPU's can have higher memory bandwidth, and can give you fast enough processing and high enough memory/data throughput to help an SLI system, IF they are overclocked...

If you run stock CPU settings, and load it up with dual cards, it will in most cases, be slower than a single graphics card, due to the demand put on the CPU and databus/chipset, to handle the extra processing/data.. However if you overclock your p4 "FSB" you can get close to the same results as a 2.4 to 2.6 ghz core duo; my e6600 was too slow at stock speeds for dual 8800gts's

You probably know this, but I'll just say that my CPU is a dual core and supports HTT. Also, my PSU is a Generation II Viotek commercial grade 550W.
I'm drawing a blank on that PSU, is it good?

If you have the white papers, and can find out if it's on your graphics cards "sli certified" list of PSU's, or if it has enough amperage on the individual voltage lines to handle all your parts power requirements...... It should work. Most of the builders that I've talked with say atleast a 600w PSU to start with on new systems that include 38xx series or higer/faster, nvidia 8800/faster/higher etc.. I've seen people using high end PSU's that are rated at 500 watts "RMS" power.. This isn't peak load, and it doesn't say anything about how many amps are availabe for peak load on the 12 volt/5 volt/3 volt lines, but it still worked.