Present day amd(phenom) processors does not need DDR3 but it can be recommended considered an upcoming technology and falling DDR3 prices.
But the higher end intel i7 or stronger processors need DDR3 as a requirement.
Considering DDR2 800 and DDR3 1333 the performance difference is minimal (2~3%) in most applications.Still you can over clock your DDR2 ram to push it's performance.
If you are considering performance over price i would recommend more memory of DDR2 with higher speeds than lesser memory with lower speeds of DDR3 technology.
The actual latencies of DDR3 are almost same or a few less than the DDR2.
Also if you want huge amounts of memory only consider the DDR3.
If you are still doubt full or need help in choosing between DDR2 and DDR3
this website is good.