You've a nice rig...
The thing i would upgrade if and when the time calls is the OS, 32bit won't take full advantage of the 4GB of Ram (think it will only register 3.5) where as 64bit would handle whatever you could fit in there (some boards take upto 16GB)and im sure the quad core would love the extra space, it also depends on what else you use the system for as 64bit OS can cause compatability problems with games and such (something you may want to checkout first).
Second would be to upgrade the chip, third would be the graphics card...

but all these are only if wanted to upgrade as your system is quite high spec, and the cost of the upgrade might not show that much of an improvement so i'd just see how you get along with it as is....