This isn't the only way to increase your Macbook's storage space, but an intrepid fanboy has managed to fit his MBP with a RAID 0 setup using two 320GB 5,400rpm hard drives. Sure, his laptop is all Frankenstein looking now (the keyboard doesn't fit right, the lappie is noticeably louder, etc), but performance-wise he's enjoying quite the speed bump (and practically no battery life). The hack works on MacBooks, MBPs and PowerBooks.
RO: It's hard to quantify the effects of a RAID 0 system. Some would argue that it's not even RAID - you're taking a second hard drive and basically bolting its capacity onto the end of another hard drive. Chief advantage: a ton more space on the filesystem. Chief disadvantage: if either of the drives goes down, you're basically screwed.
Whether or not you get a speed boost has to do with how reads and writes are distributed across each drive. With a system like RAID 1 (mirrored) it's a lot easier to predict, because both volumes are supposed to end up identical. With RAID 0, you have to take into account what data is sitting where.
Anyway, I don't really consider myself an expert on RAID performance, but the folks at AnandTech did some testing of a packaged RAID 0 system a while back, and... well, they didn't seem too impressed.