Is a Radeon 5850 Overkill for My Application?

i'm putting together a pc build for to replace our 4 year old dell which just isn't up to the task any more. primary use (other than normal home/office and internet use) is working with large raw image files, maybe fooling around a big with video but on an amateur basis. gaming will be on the light side. i've spent money on graphics cards in the past and am wondering how powerful a card i need for this build. my current thinking is a radeon 5850, but if that is a dumb idea in either direction i'd like to hear about it. most of the extra money i'm spending on my build is on fast and large storage (80gb ssd, 2 tb hdd). the rest is pretty standard--core i7 930, 6gb ram, haf 922 case, etc.

i'd also appreciate advice on whether i would notice performance benefits from overclocking the cpu a bit. the reason i hesitate is that if i overclock i guess i also need to upgrade cooling which adds to the cost..and i'm already up against what i want to spend on the pc so am trying to go the other way if possible.

thanks!!

i use a dell 2209wa e-ips monitor. some chance i'll go a bit bigger at some point but i really like this monitor's performance. when i switched to this monitor about a year ago i really noticed a big difference in what i was seeing....