both are alot! but atm the 7800 isn't needed in current games... it's the best gfxcard out by far! and the most futureproof ... i would go with the 7800 for the time being... just for the bragging rights but it is around £100+ more!
If you want to spend that much more money on a gfx card that's the one to get (7800) but the X850 Xt is a brilliant card... whats your systems specs??
value for money the X850XT is the best, performance the 7800 will win hands down but as G~man said there are no games that the 7800 is really needed for, best price on the X850XT is around £300 the 7800's are pushing £4-500
WOW ! lol just looking at the specs of the 7800gt and it needs a minimum power supply of 400W ... and if you want to go SLi you will need a minimum of a 500W PSU! ... just fort id give it a mention
and the best value for money ATM is the X850XT... it should last a good couple of years but if you feel you need to go one better i would say the 7800gt.. i assume you can have everything full whack on these things and they will run with no problem at all!
Once the next gen of console come out, you can expect the need for a more powerful computer to play game at full potential to be in need as well!
also there is one game even SLI can barely run... EverQuest 2... not even a 6800 Ultra in SLI with 2 Gig Ram and FX-55 can play it on full graphics.... so ive heard