imo you need a video upgrade and ram upgrade the most, considering that your video is onboard and you cant upgrade the ram without bus bottlenecking it seems to me you should go with an upgrade for the motherboard if possible. a new processor would help out, but the one youve got shouldnt have too many problems, so i'd label it a "only if youve got the cash.

in summary upgrade the following, in order of importance

video
motherboard
ram
proccessor <~~~ maybe for this one.