Barring quad-SLI setups or other such trickery, there is nothing which offers higher performance than the Nvidia GTX 295. Like the Radeon 4870 X2, the GTX 295 takes two already powerful GPUs - in this case, the GTX200 GPU - and pairs them together for incredible performance. The results are even more stunning than one might initially guess thanks to multi-GPU scaling which seems to work quite well. The GTX 295 has an average FPS of about 50 in Crysis Warhead running at 1920x1200, which is 10 more than the average of the Radeon 4870 X2. The difference is subjectively noticeable, resulting in smoother gameplay.
Of course, you get what you pay for. The GTX 295 runs $500 at most retailers. Sales and mail-in rebates are not that common, either. In fact, those who are interested in this performance, own an SLI capable motherboard, and wish to save some cash would be wise to buy two GTX 260s rather than the GTX 295 as they perform identically but cost about $200 less. That said, there are reasons one might want to run a single card solution, and in that case the GTX 295 is the best money can buy.