What about RAID 0+1 with 4 hard-drives?
I would 'think' this would result in the same speed as single harddrive. I can't prove that, but any speed you gain off of RAID 0 would be lost in the RAID 1 parity. It's a nice setup for mission critical storage so that if any single harddrive failed you would loose all your data...but that's not going to help you. RAID 1 and RAID 5 SLOWWW things down--that's the cost making sure a harddrive failure won't result in a lost of data.
Follow mine and cisco's advice: Get a 10,000 RPM drive. Forget about RAID 0. I know all the cool kids have a 'RAID' on their system....but it will not dramatically increase your speed (as shown in the Storage Review article).
You may think that 37GB, 74GB, or 150GB's isn't a whole lot of space. 74GB is PLENTY for installing an Operating system and MULTIPLE games. For all your Music, Movies, Data (game patches, images, etc), get a second, larger but slower, harddrive. 320GB 7200RPM drives are $90 on newegg. A 500GB 7200RPM drive is $150 on NewEgg.