The 7800gt is a bigger and hotter chip with limited overclocking. The 7900gs is newer, smaller, and much cooler running than the 7800gt, it also overclocks like a beast. Where on the 7800gt you are only likely to see the core hit in the low to mid 500's and the RAM hit mid 1200's the 7900gs will easily do over 600 on the core and over 1650 on the RAM, no volt mod needed. I personally had a 7900gs from the woot.com deal running 700/1900 with a volt mod and a VF700. The most I could get my old 7800gt up to was 550/1300 even volt modded. They both have exactly the same features bit the 7900gs's are going for like 165-170 new and 140-150 used so they are the much better buy right now.
morgash