No, Steampowered will not be any different. That is where I got my game from. I can say though I am running win 7 on my computer. I believe the graphics card was a GTX 9800. This computer runs well but I can’t remember what card I have in it.
Most of my problems worked out with updates from Microsoft, I think, some by fiddling around with drivers and settings. I really think your problem stems from a lightweight graphics card. The 9500 I had before ran Fallout 3 pretty well after lots of updates and tweaking, but it was just too much for anything over medium settings.