Re: What to upgrade....
Not going to be able to crossfire the 4650 with a 4850 you would need both to be the same. Iím not sure what your budget is but I would consider an Nvidia 460 even one with 768 Mb as opposed to the 1 Gb version would be better than the 4850 and you could get that for a decent enough price.
Game - BIOSTAR TA785 A2+ / Phenom II X4 965 Black Edition Deneb 3.4GHz 125W / Patroit 8GB DDR2 800 CAS 4 Timing: 4-4-4-12 / Sapphire HD 6870 1GB / Antec Three Hundred Case / PC Power & Cooling S61EPS 610W / X-Fi XtremeGamer / Western Digital 640GB 7200 RPM SATA 3.0Gb/s /Windows Vista home premium 64 SP1
Back up - Biostar 6100-939 / A64 3800 Venice(Stock 2.4) / 1.5 GB Corsair XMS 3200 / HD3870