Ahhh mate.... I had this problem and my specs were similar to yours - but unfortunately it might not help.
Processor: q6600 2.4
MB: several. both asus and gigabyte
Ram: several. corsair and kingston (1 was 2x2gig, 1 was 4x1gig 800mhz)
HDD: several. Samsung, WD, Seagate
case/psu: Antec Sonata III w/ 500w psu
As you can see, I tested alot of hardware to try to solve the problem. 3 HDDs, 5 mobos (2 models of board), 1 replacement case and psu and 3 pairs of memory sticks (two different brands). The entire PC was also sent to ebuyer (component retailer), twice, and both times they failed to find the solution. I even left it with a specialist PC store for 3 months. The reason why they were both hopeless is that it was such an intermitant problem, so finding the culprit is very difficult.
Not trying to depress you but it sounds like you have something similar. (Don't worry it appears to be fixed now - 1 year later!)
I've got a few questions:
1. Are you sure its sleep mode? That's what I assumed too. It might not be. Turn off sleep mode and leave your PC on for a day (or 3 - mine could be stable for up to a week). What was happening on mine is that the system was restarting and then hanging just after the logon screen - but it took me weeks before I actually worked that out because my monitor would instantly go into power saving mode, making it appear like a sleep mode that I couldn't wake.
2. After it has crashed and you restart the system does it always boot? Or does it sometimes hang?
3. Have you had any error messages from Vista?
I'm no expert at all mate, but I spent a **** long time on this (on and off for a year). I initially thought it was a memory problem, but I tried XP with 1 gig of ram (alternating 4 sticks), I performed a memory test, and ultimately a new brand.
If you've got something similar to me then I very much doubt you'll be able to google an answer - i've looked for ages.
First off - does your graphics card have two power cables? Are they plugged into two different power cables from the psu? (ie not using two molex plugs from the same cable). Might seem odd, but this is one of the only differences between my old unstable PC and the stable variety.
Hope this helps.
Nick
PS: basically the specialist pc place admitted they weren't sure, but suggested replacing the MB and gfx card. Seems to have worked. Depending on your consumer laws/rights you might be able to do what I did and send it back saying its not what you want and choose another model. That is if you have the same problem!