I'd suggest if you go ATi, then get Sapphire or ATi brand. They seem to have the best compatibility with the official drivers, also tend to perform much better than the 3rd parties.
This said, if I hadn't had the urge to buy a HD 2800 XT from Sapphire directly; probably be waiting for the HD 2600 Pro to come out right now.
Performance wise, NVIDIA is owning with quite a margin; even with the new HD-Series just hitting the stores NVIDIA's last two generations have had much more power behind them. They're more stable, and generally better drivers.
I switched to ATI when the X1K-Series was released, now using them fairly exclusively. This is more down to work demands than much else though. I've recently fallen out of favour with NVIDIA, because of a simple device ... Playstation 3. What possibly makes it worse is NVIDIA actually had a deal with Microsoft that they would be the only manufacturer allowed a DirectX10 card for 3months after Vista release, and they were the ones who decided on the Shader 4.0 specification. ATi now is starting to feel like the underdog in all of this, and their prices are also far more competitive.
GeForce 8800 GTS - £250 or Radeon HD 2800 XT - £210
Although sure the GTS boasts 384MB DDR2, tbh that's just ridiculously stupid amounts of video ram; and with DirectX9.0c or DirectX10 is totally wasted given you can now scale back what the graphics card uses entirely down to requireing on the framebuffer. This means at most 1080i 8xAA 16xAF, which requires roughly 32MB Frame Buffer.
The rest of the 3d stuff can be streamed via the CPU or RAM using the 512-bit Address Bus at 16x(Bus Speed)Mbit/second, which ends up being NO different to using the ram on the card itself. Honestly if they scaled back and forced developers for the PC to use the system ram like you would on a console, then it would slash the card prices given most of the production costs are memory!!
Hell that's why TurboCache and Hypermemory work they way they do, only rather than being developers supporting this method; ATI do it via the driver software at a small performance loss. Goes to show we're just paying the odds for something that honestly isn't needed anymore. Wish ATI would push for their entire range to go with Hyper-Memory, and actively support developers to use their method of memory management. We do it on consoles already, why should we be so wasteful on the desktop simply because we don't have restrictions? It's those lack of restrictions that mean the Unreal 3 engine runs perfectly fine on a CPU capable of similar performance to a Core 2 Duo 1.0GHz and quarter the entire memory available on most current desktops... yet require top-end Windows sytems to run. Yes you loose some performance because of the Windows desktop running more background applications, but this is roughly 5% not 250%!!
In closing, go for whichever is best in your price range; or wait for the next generation cards to release their mid-range over the next month or so.
