Try with the latest drivers, then you can rollback if they aren't any better. Vendors fix video card issues all the time in driver updates.
Unsure how running the card in a PCIe 1.1 slot instead of a 2.1 slot would effect it. Your mother board appears to have both PCIe slots as 16 channel though, so it shouldn't be too bad unless that's just an issue with that particular card I think.
Doubt it's a power issue, 600W power supply should be more than enough. If the card is designed to get all of its power through the PCIe slot, it should get all the power needed through it.
After you try updating to the lastest drivers, try running Spelunky 1.2
http://www.spelunkylevels.tk/spelunky12/downloadtracker.php?file=spelunky_1.2_July_3_2012.zip - it's compiled with a newer version of GameMaker, might have better graphics compatibility.