All newer PCIe cards are backward compatible with older PCIe slotted motherboards (with a few rare exceptions... but not to worry).
If your 450W PSU has 2 6-pin PCIe power cables and is of decent quality, it should be sufficient for even the GTX 260. (See below)
The processor and RAM are no issue.
Here is a PSU calculator if you want to plug in your system's values.http://www.thermaltake.outervision.com/
... but I think you will be fine.
Also, here is a graphics card hierarchy chart for comparisons of cards.http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html