um no...ATI currently has no card that ups nvidia cards...why? have you seen an ATI card with SM3.0 yet? i dont think so...ok, Sm3.0 isn't that big of a deal, you say...it is though, as next gen games will most likely run in Sm3.0 only (either that or very bad looking 2.0 with decent speeds, or good looking 2.0 with bad fps)...without HDR, Sm3.0 looks better and runs better than 2.0...have you seen the newest splinter cell game? A 6600gt runs Sm3.0 (without hdr) smoothly...in fact, it probably gets a constant 50fps (prolly around 30 on 10x7)
Nvidia cards are more future oriented...when UE3 games come out, even if the 6800 runs slow on them (which it is said will run easily), the x850 will run even slow (and look worse)...if you upgrade often, usually ATI has better now cards and usually better future cards, but in this case, to be future proof, you'd want a 6800gt/ultra.