Re: ati 3650 vs ati 3850
I don't know the brand name of your PSU, so I'm assuming its probably not very good.
The HD 3850 is easily better and pummels the HD 3650 (I own the former).
The HD 3650 is more efficient for your rig. However, the HD 3850 AGP requires a lot of power to run properly, most of which older PSU's have trouble with, and even if you got the HD 3850 AGP to work, it'd be severely bottlenecked by that Xp 3200+
I'd say forget buying either, start saving up for a bit of a newer rig.