Your current PSU : 300W - you need to upgrade, 550W or more, need lots of power on +12V rail.
Your current GPU : HD4850 by 6970, good choice,
Your current CPU : Q6600 - Still a very good CPU for gaming
if you made a plan to upgrade your gpu then you must upgrade your cpu as well your cpu is able to run upcomming games but not on max setting your cpu is the starting model in quad series and not suitable for latest cards.
Suppose if you buy a heavy card like ati 6970 or nvidia gtx 580 may be your cpu has huge bottleneck and gamming performance may you lost.so you should go for q9550 or i5 2400 iam sure you want a card for heavy games and you playing games at max setting iam i right
It could probably play it and quite good settings, but your CPU would be your bottleneck. If you upgrade to a better Core 2 Quad series, or overclock (prolly can't do that if you've got the stock Dell motherboard), it would help things out a bit, but I do believe you could play decent games with the Q6600.