I would foremost recommend a RAM-upgrade if those numbers are correct. I'd say the new Corsair Vengeance 4GB-modules, really cheap due to new technology (factory wise). A bump in the graphics might prove prudent as well, certainly now when nVidia is bashing forwards with new technologies specific to your area of usage. I have two GTX 470 and although I don't do much rendering myself, they fold proteins like crazy (Folding@Home, if you don't know about it, try it out today:ref google) and I've watched videos of their (almost) real-time photo rendering. GTX 460 is a great card well worth the low buck!
But I'll tell you something that may seem easy, but will probably be the cheapest boost you can make; buy a decent cooler for you CPU and dash it up to a blazing 3.8GHz instead of 2.8GHz
I can recommend the well-known Corsair (Oh yes, I dig Corsair) H50 for that.