you using a 20pin or 24pin PSU?
i forget how those work cause i dont own one, but if you dont have a 24pin psu you need the PCX power connector pluged into your 6800 vidcard.
you wont beable to run games properly without pagingfiles, check pagingfiles settings at....
/control pannel/system/advanced/performanc setting/advanced/virtual mem (if you have 1gbram, set paging to 1024min/max)
you didnt over-clock anything did you?