I was looking at Extremetech.com's review of the eVGA 7800GS Superclock and compared it to THG VGA listing for the ATI x850XT PE. Comparing the two different benchmarks shows the 7800GS SC clearly better than the x850XT PE in performance. One must remember two things here. 1) The 7800GS SC is an overclocked card and not a regular reference card and 2) The benchmarks, properly done in their own setting, are not done in a direct head-to-head match up by the same person/s in the same exact setup and lab conditions. Still, if one is to believe the benchmarks are fairly well done and fairly accurate one could still conclude the 7800GS SC the better performer. At least until someone can put them in a direct head-to-head benchmark setup. Not sure which card is the cheaper to buy. I didnt price the x850XT PE but the eVGA 7800GS SC is going for $314-$340 online.
To the OP,
You'll have to decide wether the 7800GS SC is worth the extra $100 or so you'll need to shell out after selling your x850XT to your brother. The 6800 he's giving you will probably cover that extra cost though.