~80%.. of which 60 to 70% is used by the game, the other 20% is just sorta idling empty

The card should be more than enough to drive that screen. It's roughly equivalent to 1600x1200 in resource requirements.

The probable thing to look for would be bad video drivers, lots of complaints on the 9x series with out of date drivers. If not, my guess is the memory. 2 gigs is a little light on the ram for a Vista ultimate gaming computer. Sins burns 700+ megs just starting up, with so little to spare you're really chewing through the stuff. Your page file access is probably quite heavy.

If you have a fairly fragmented install, that memory usage would be a severe problem. Fragmentation and high page file access do not mix. You could have twice the computer and it would still run like shit if the game files were scattered across several thousand fragments.

Although... Core 2 duo at 4ghz? If you actually have an E6550 processor, there's something wrong with your description. The Conroe is in the 2ghz range, either you're severely overclocked and the heavy processor use from sins is trying to melt your cpu's, or you're adding the cores together, a no no. Assuming you gave the right processor, either case is also a potential problem. Sins can be rather cpu intensive in the late game. Your description of right from the start crap performance doesn't relate real well to a cpu bottleneck, but I've got a couple 3.2ghz Xeons here and I see 40% utilization from startup so maybe.