I will say, BEFORE the DirectX 9 update, EQ seems to like Nvidia cards more than ATI. I went from GeForce 3 to an ATI Radeon 9800 and didn't see much difference at all(ATI card is 2 years newer (aka 2 generations as it's comparable to Nvidia's GeForce FX (which is technically GeForce 5))) so I felt their should have been a reasonable difference. I'm sure the CPU you run is a factor too, I was running a 1.7Ghz P4 with 2 120gig Maxtor 8MB Cache drives in a Raid 0 config. I update more recently to 2.6 Ghz CPU and saw a nice difference, but I think it was more because of the CPU than the card though I'm sure they are both a factor.
Since the DirectX 9 update, I couldn't tell you if it prefers one vid. card over the other anymore, but I'd still guess Nvidia is the favored(difference games seem to run better on different cards, I think part of the reason is the machines the game is coded on and then later support for other cards is added and/or certain cards just run certain code better).
I'm running 1GIG PC 3200 on the 2.6, but had 640MB of PC 800 (Rombus DRam) on the 1.7. EQ is a ram hog so more is better.
I recommend you build a custom PC myself and feel you'll get more bang for the buck than with a pre-built system from a big name. You'll only need a Case(w/powersupply), Motherboard, CPU and Ram. Unless your old stuff is REALLY outdated, you can probably get by with your old drives, vid and soundcard. Depending on how much you spend, you can always update the other stuff later as needed, but this will make the biggest difference and should only cost you a few hundred.
Case will cost you about $50 if you stay simple(can still get a nice looking case for that even), Motherboard should run $150ish(I recommend ASUS P4C800-E, Ram about $50-90 for 512 depending on the standard(I'm figuring on PC3200 for the ASUS board) and then get a reasonable CPU(I recommend 2.6 P4 for the money. You can always update to a 3.2 and possibly higher later, this motherboard is really good, has built in sound(I'm running Audigy though) and Serial ATA support and even has a built-in RAID controller for SATA or standard IDE drives.).
You'll fall within the $600 budget with the rig I'm telling you about and have room to upgrade to a better CPU later without needing to change your Ram or Motherboard, supports RAID for even more performance later.
WARNING: In about a year, maybe a little sooner, their will be a new PCI standard coming out and PCI slots will be replaced with PCI Express x1 slots. Your AGP slot will be replaced by a PCI Express x16 slot. Your old PCI cards will still work, but AGP will be gone. Keep this in mind before buying a $400 video card that you might have wanted to keep for 2-3 years, your new system probably won't be able to use it. Also, if you are looking into Serial ATA drives or controllers, 2005 the data rate will double, Currently 1.5 Gbps compared to IDE's 1.08Gbps, Serial ATA will go to 3Gbps in 2005, 6Gbps in 2008 and 12 Gbps in 2012 (Raid0 you'll run double those speeds(I believe those are burst rates though, much like with IDE's max data rate))
Unfortunately, standards are always changing. The bottom line is though, you have to jump in eventually.