I'm building a new system and I'm in doubt between buying one 500GB hard drive or getting 2 - 250GB...
How much faster will my computer be and also will that give me a better performance in games aswel??
What do you plan on doing with the 500GB? Do you want performance, redunacny, or just looking to see how green the grass is?
Using 500GB and dumping your OS there isn't much of a design. Typically if you want performance on the dollar you go with a straight Raptor for your OS and use a larger drive for storage. Read up on the threads on what you can get from raid (2 Raptor or whatever) in a raid 0. If you understand the levels of raid and how the combination of combined disk resources for a particular raid level benefit a particular needs, you'll know if it's worth it for you. But you haven't expressed any needs, only a vague want.
I think you'll be happier with getting one 10K HDD for OS and games and one 7.2K large HDD for storage, as opposed to setting up a RAID0. My friend actually gets faster results using a single 10K HDD compared to two 7.2K in striped configuration.
As always, it depends on the use. For most uses, there might not be much difference at all. Faster harddrives only pay off when your doing tasks that access the harddrive. (as much as I try to tell people, GAMES don't qualify.) If you setup an AID0 array on the 2 250GB drives, you might get the game levels to load 2 or 3 seconds faster. (depending on the game, it might be less then one second faster...)
I'd keep it simple and just get the 500GB drive. No RAID drivers to get corrupted, and its just the single drive. (less heat, noise, electricity, etc) RAID is for heavy use servers, not single use desktops.