I don't understand cache regardong photoshop cs5. I am buyibg a new PC and I need a recommendation for a graphics card. RE Hardrive, should I get one or two. i eill do photoshop and later video editing. and I want everything to move FAST! thanks
Video editing or any rendering benefits from two or more hard drives. This way you can read from one and write from the other. An ssd would solve that problem, but the capacities aren't there yet for a reasonable amount of money. I would snag a couple of f3's or a couple WD blacks.
Some parts of CS5 benefit from GPU acceleration. I don't know if it works with ATI graphics parts; I know CS5 uses the Nvidia 8600 in my notebook.
Look to get a mid-range Nvidia GPU. You don't need to spend a ton or have the latest and greatest; a GTS 250 should do you. If money is no object, you can get something like a GTX 260 or one of the new Fermi cards (GTX 470 and 480).
Definitely have two hard drives. As sportsfanboy said, the Samsung Spinpoint F3 and Western Digital Caviar Black are good choices. Buy two of the same size, and make sure your motherboard supports RAID.
You could also buy a third HDD just for your operating system if you want to keep your RAID clear for video editing. An SSD would be a good, but expensive choice for a boot disk. This is expensive, though, so again... only if money is no problem.
It almost goes without saying that you should gave a quad-core (or 6-core) CPU and as much RAM as you can afford for video. Try for 8GB, four if you're on a budget and 12 (or more!) if you have extra cash. Intel's six-core CPU is the fastest thing right now, but they run about 1K USD. AMD has a slower six-core offering in the $300 range. Both would be great for video; so would most quad-cores out there. Buy what your budget can handle.