go with the gtx, it'll do fine by itself for a while and when you add another i'd say it'll last you almost until the next windows platform comes out. gts int really worth it, they look good now but when theres more cards out with dx10 they'll be crap in comparison. gtx will be high end for quite some time, look at the 7800gtx, still runs everything fine at high res
why?..... SLI or Crossfire for that matter.... barely more than a marketing ploy to get ppl to buy 2 video cards.
why? driver support for SLI is terrible if not criminal, very few games are supported which generally causes the system to default to 1 video card or perform worse or become unstable which has happened...... this would be ok ..... well not really but what makes it even more scandalous is that Nvidia stops tweaking performance on current generation cards once next generation cards are released..... Crossfire is little better and less mature from ATI but at least with ATI you get longer driver support.... the reason can be seen in the longer lived GPU architecture's that ATI tweaks over multiple generations compared to Nvidia which prefers to start from scratch with each series IE: 2, 3/4, 5, 6/7, 8..... while ATI went 7, 8 / 9 / X8, X1000/R600 which is an evolution of X1000.
optimising drivers for applications is expensive while SLI and Crossfire both require drivers optimisations in each application to recieve notable improvements... SLI and Crossfire owners make up less than 10% of the high end market which amounts to less than 5% of the enthusiast market which again represents in the neighborhood of 10% of the entire market.... a niche within a niche within a niche as it were and you can see why both companies have followed the same path, optimise for popular games that are benchmarked by websites and optimise the rest eventually depending on when the next generation comes out.
go to Tom's VGA Charts and compare older generation SLI to next gen hardware the tale is always the same the next gen spanks the SLI setups.... so would I reccomend anyone go SLI..... sure if they didn't care about cost and didn't plan on using it any longer than a single card going in with the full knowledge that the next generation cards will decimate the SLI setup when new applications arrive..... along with that the newer features that come with new gen cards and better DX support with each gen should also be accepted and understood ahead of time.
IMHO I wouldn't reccomend SLI or Crossfire to anyone it's a marketting ploy to sell more video cards and little else... Nvidia forces SLI support to be Nvidia mobo's only because their is so little to be gained by making it widely accessible so they try to get additional coin from motherboard sales to justify it's existence.
just prior to AMD's purchase of ATI, Intel was planning on launching crossfire supporting motherboards as ATI Crossfire was barely making inroads it would have been a win win while Nvidia would have been left vulnerable but things happened and now Intel is in an uneasy/enemy of my enemy alliance with a Nvidia until they can get their graphics ambitions in place.
Neither - wait until ATI release the R600, then you'll get a better idea of the performance.
The other potential problem is when you try to run DX9 games on Vista - there's no support in Vista for DX9 other than a software "interpreter", so in all likelihood DX9 will be slower under Vista than XP on the 8800/DX10 cards.