Possible Upgrade

I'm thinking about possibly upgrading my video card/sound card in the near future. Of course, I want to make sure that both components work just fine with Vista. The plan is to get a decent DX10 video card that will last roughly 2 years. I want to be able to run g***s at no more than 1280x1024. As for sound, I have a 4.1 sound setup. I would like my budget between the 2 components to be no more than $400 if possible, I may be willing to bend a bit if I have to. Waiting to build a new system isn't an option, as I plan to do that in about another year and a half anyways. I really just want something that will hold me over really well for the next year and a half or so. Here is what I currently have in my system:

Asus A8N-SLI Deluxe
AMD Athlon 64 X2 4400+
2 GB PC3200 Corsair Value Select
250GB Western Digital SATA 3.0GB/s
2 Nvidia 6800 GS cards in SLI
SB Live! 5.1 PCI
Enermax 530w PSU

Based on those specs, what would you recommend I get?

PS- I know I currently have SLI, but I don't want another SLI config, a single card with 1 GPU will be just fine. I play g***s, (WoW) mainly, but also play FPSes such as CS:Source, and I'll prob give future g***s a shot (UT2007, Crysis, etc.). I don't need ultra high resolutions, only 1280x1024, and AA and AF is nice, although not required. Thanks in advance!