The reason your games are choppy is because of your graphics card, not the processor/mobo/RAM. The Phenom II X4 955 is still plenty of processor for gaming. The difference between DDR2 and DDR3 is only noticeable in benchmarks, not in gaming.
Windows 7 is fine, though if you're doing ok on Vista, there's no super need to upgrade.
SSDs are very nice, but won't improve your gaming performance outside of load times.
I'd say that you should prioritize getting a new graphics card. Then save up for an SSD and Win 7, since you're going to have to do an OS load on the new SSD, you might as well upgrade the OS at that time too. Don't bother upgrading cpu/mobo/RAM for at least another year, maybe more.