That's a tough call for me. On paper, the 512MB DDR3 is the better card. Trouble is, it's a Diamond, which means (based on my bad experiences with the brand) I wouldn't touch it with with a ten-foot pole.
If your PSU is reasonable, and if you might play any 3D games at all, you might consider a 3850: http://www.newegg.com/Product/Product.aspx?Item=N82E168....
At first, I thought that a 3850 wd be a *lesser* model - due to the name value of the model - but I just checked Tom's GPU hier. chart, and found that a 3850 is actually rated higher than the 4650, and it is equal to or even less than the price I was looking at with the 4650.
EDIT: The maker is called Sapphire. anyone know whether this make is good ? (I saw the bad experience with diamond above).
also, when the reqs say that a 450 psu is needed, how is the other devices and load taken into acct? what does the req assume is connected to the psu, a mobo (what kind?; there are many), cpu (what kind?; there are many cpu's), one optical and one HD? What if you have more than one HD?
my 460 watts will be connected to
a p4 3.0 cpu
a dell xps gen 2 something mobo
an ide dvd writer
at least two , and maybe-probably 4 , 5, or 6 sata HDs