greenmedic88 on 03 February 2010
Nvidia cards can only SLI with the same cards. The 9800GTX (GTS 250) and GTX285 (still overpriced) are from two separate generations of GPUs. The 9800GTX based on the G92 GPU is actually a modified 8800GT.
Buy another GTS 250 (and buy it cheap) if you want SLI performance for the least amount of coin.
Crossfire allows ATI cards to mix and match to a degree (same generation), but the down side is memory will clock down to the lowest common denominator (slower card). Always better to match same cards whenever possible.
