Here is the official system requirements for the HD 4850 graphics card. Note the power requirement. You should be OK with your existing PSU, however you will be running fairly near its maximum rating with very little room, if any, for further system expansion. I suggest that you upgrade your PSU to at least 650 Watts.
(Note: Because this is an image, if you wish to follow the link in green, you will need to type the address manually.)
4 x 4GB DDR3-1600 Corsair Vengeance CMZ8GX3M2A1600C9B (16GB)
Graphics card(s)
MSI GeForce GTX770 Gaming OC 2GB
Sound Card
Realtek High Definition on board solution (ALC 898)
Monitor(s) Displays
ViewSonic VA1912w Widescreen
Screen Resolution
1440x900
Hard Drives
OCZ Agility 3 120GB SATA III x2 (RAID 0)
Samsung HD501LJ 500GB SATA II x2
Hitachi HDS721010CLA332 1TB SATA II
Iomega 1.5TB Ext USB 2.0
WD 2.0TB Ext USB 3.0
I ran that card - MSI R4850 - just fine with a 400W Fortron Zen fanless PSU albeit with a low power AMD 4850e (no relation!) processor. PSU got a little warm but no problems.
Cheers guys it arrived today! Very happy with the performance so far tested on Crysis Warhead and PES2009 Demo. How do I check the temperature of the card? I used Everest Ultimate Edition to check for my old 8600GT but it doesn't show for my new card.
EDIT: Also both PES2008 and the PES2009 demo are crashing when running in windowed mode. I never had this problem before. Anyone know how to fix this?
in catalyst control center there is overclocking area at the bottom of the app that shows you the temp or you can just download gpuz GPU-Z Video card GPU Information Utility
there are alot of fixes in 8.9 that were causing me problems in 8.8 so be sure thats what you want to do before you do it the 8.9 drivers are a lot better too an fix some of the Anti ailiasing problems i have running dx 10 games running it in full screen mode is the prefered option for most games to it may be that the game just doesn't like it that much also you could look for updates to your game which can resolve alot of issues too.
Pro Evo is the only game I run in windowed mode because I don't have a widescreen monitor so it look better in a widescreen resolution like 1280x768 windowed mode. I never had problem with it before changing the graphics card but it looks like I will have to put up with it for now! I have tried the 8.8 drivers but now gone back to the 8.9. The card is great with Crysis and hopefully some of the new games due to come out in November now. Thanks for the help mate.