I am thinking about purchasing a new hard drive, I have a HP pavilion with an ASUS CUW-AM mobo. Now im not sure what the difference is between SATA, IDE, RAID etc. which one should I choose? and is there any thing else I should take into consideration before buying one(aside from the size)?

IDE is good old classic connection, and as far as I know all mobo's support this. There may be a few exceptions for very very very uber leet new ones however.

SATA is much quicker than IDE, but not all mobo's support it, and it does add to the price tag. Only go for SATA if your looking for quick loading times for apps.

SATA is much quicker than IDE

It's actually not much quicker at all (yet!), and the performance difference is hardly noticeable. You might be thinking of the 10,000RPM drives. ;)

I'd only choose SATA for the smaller cables, easier physical installation (no jumpers) and some degree of future-proofing. Considering SATA models don't cost much more than their IDE counterparts, why not? Unless of course your motherboard doesn't actually support them. :)