Upgrade worth it?

Hey I built a new PC a few years back and was wondering if I should upgrade it or just build a new PC.
Current Specs
Q6600 intel processor o/c to 3.3ghz
650 w corsair psu
4 gig ddr 2 ram
no video card (My 4870 1 gig died on me. My fault, I tipped over the computer case while I was working on it and it fell right on to a glass of coke )
640 gig hard drive at 7200 rpm
Currently on a 1080p 23" monitor
OS: dual boot Windows 7 beta 64 bit and Windows Vista Ultimate 64 bit

Possible Upgrade
4890(not sure between sapphire or XFX)
4 more gigs of ddr3 ram(total 8 gig. Again, I am not sure this will make that much of difference and I am open to suggestions)
or 8gigs of ddr3 ram (I have one of those useless motherboards that supports both ddr3 and ddr2)

or should I just build a new I7 build, because my processor is pretty old and I don't know how much longer I can continue on with it. (I need a rig that lasts me for a little less than 1.5 years, and I doubt I can do it with my current specs)

Sorry I meant 4 gigs more of ddr2 not ddr3 or 8gig of new ddr3 typo
Anyway thanks for the quick reply boonality, and I will probably go with the 2x2 sticks of ddr2 like you suggested if I choose to upgrade.