already using the gpu client for my gtx280 just duno how to set it up for 2nd GPU in the a 8800GT
It may not be possible or worth the effort:
The client's cores currently don't support cards with different shader counts (or different core architechtures) very well, it is something to do with the way the multiple cores use the driver.
Essentially this causes something to go wrong - one card will either run using only the number of shaders on the lower spec card (i.e. the gtx280 would only run on 112 shaders rather than its full 240) or one card just spews EUEs and doesn't actually do anything useful (your gtx280 would run at full pelt, the 8800 woudl just sit throwing unstable machine errors).
This has supposedly been fixed in the fahcore_11 BETA that is buired somewhere on the folding site (it is v1.31 rather than the 1.19 core that the client automatically downloads) but i havent had the setup to test if it has genuinely been fixed. (Tried a while back with a gtx295 and a 9800gt)
However there is no beta version of fahcore_14 so any of the 472 or 1888point work units will just fail as above.
If running Vista or W7 then you will also have to use a dummy plug or connect both cards to a monitor and extend the desktop across this pretend screen, to trick windows into allowing the card to act -this is not an issue in XP. there is a guide on the main website.
Multi-card setups work best when all cards are of the same model with equal shader counts, as it circumvents this issue.
There are new clients and cores due out in the next few months, hopefully ths will be addressed properly in these and you can fold on any combination... we will see!