You seem awfully stubborn and optimistic about your 8500GT. It's not a powerful card at all, and yet you expect it to run the most graphically demanding game on the market "fine"? Sorry, not gonna happen, unless you don't mine turning a LOT of stuff down. I don't know where you got the notion that it's a good card for Crysis.
The best bet for a 300W PSU is the Radeon HD 4670, which uses surprisingly little power for a decent card. It'll run Crysis with medium-ish settings at maybe 1024x760. That's a whole lot better than the 8500GT, no question (with GeForce cards, x400 or x500 GT cards are considered the budget line, as opposed to the x600 or x800 GT's).