I just upgraded my 7800GTX 256mb card to an X1900XT, mainly for the 512mb frame buffer. If you want to run at 1200x1600 or 1200x1920 resolutions, you'll really want a large buffer. With a mail-in rebate and shipping costs, the total came to around $425. For newer games like Oblivion and F.E.A.R., it was well worth the upgrade. And with the Chuck patch, I can run Oblivion with HDR and AA enabled -- something nVidia cards cannot do. Even though I purchased the 7800GTX only 7-8 months ago, I'm very satisfied with the upgrade. I loved my 7800GTX, but ATi has really made something special with the X1900XT/X cards, and you can actually find one in stock.
Compared to my "old" 7800GTX (256mb) card, I get around a 20-50% increase in framerate. Keep in mind that I enable VSync for all my games, so the gain in performance could be higher.
If you're not going to invest in a fast CPU (AMD FX-55 or Intel equivalent), dual-graphics cards might not be the way to go, unless you're using 2 older cards. The CPU can become a bottleneck. Since I "settled" for an AMD64 3500+, I only went with a single-slot mobo. At the time, CF hadn't been released, and nVidia was struggling with widescreen resolutions. My next build will have 2 PCI-E slots, but as of right now, I'm very satisified with my setup.
As far as a Vista-compatible card goes... listen to the other posts. It's really not worth worrying about at this point. I'm sure the whole Win2k fiasco makes most gamers leary about upgrading to a new OS as soon as it's released. Personally, I'm figuring on giving it 4-6 months after Vista's release before I upgrade, and only if there's a compelling reason to. Halo 2 isn't one of them.
But if you're building a PC from the ground up, there are other reasons to consider waiting until the summer. AMD will be releasing their new socket design very, very soon, which will support DDR2 RAM. Also, Intel will be releasing their Conroe CPU as well. By that time, ATi and nVidia should be ready with their next batch of GPU's. And maybe we'll finally get to see if the Aegia PhysX card is all that they say it's supposed to be.