For the most part, the difference in the cache size will be pretty negligible to you. There are situations where it makes a world of difference, but normal desktop computing is not one of them.
RAID 5 is a combination of striping and checksumming, and for desktop work its the best of all situations. You need three drives to pull it off. The first two have the data striped between them, so for every file you write half goes to one drive and half goes to the other. It splits the write/read time in half because only half the data gets read/written, with two drives working in tandem. The RAID controller splits/joins the data before its passed off to the OS/program, so to Windows it just looks like you have one massive drive. This virtual drive can be partitioned however you like it, and I still recommend creating two seperate partitions, one for OS/programs, and one for data. In Windows, I map my Documents/Library folders to a folder on my D: partition, and I pretty much never have to think about it again.
The third drive is used as a checksum, and contains the data written to both drives summed together. This way, if one drive goes down, data can still be reconstructed using the remaining drive and the checksum. Its like automatic backup. If your RAID controller is decent too, it won't even crash your computer when the drive fails. It'll just pop up a warning telling you to replace a drive. When you replace the drive, the RAID array will rebuild the data for that drive on first boot, and you can continue on like nothing ever happened. If two drives blow though, you're fucked.
All this work is done by the RAID controller, so you basically have to do nothing beyond going into the RAID section of your BIOS, selecting "Create RAID 5 Array", and hitting OK. Assuming Windows 7 recognizes your motherboard's RAID controller by default (which it mostly will these days), when its ready to install it'll just see one massive drive to work on.
Just whatever you do, if you go RAID make sure all the drives you get are IDENTICAL. Same make/model/size/cache, everything, preferably. If one thing is off on one drive from the others, everything gets degraded to the lowest common denominator. If one of your drives is 50GB smaller than the others, then 50GB goes unused on each of the bigger drives. If one of the drives has a smaller cache than the others and retrieves data slower, then the other drives are constantly going to have to wait for it in order for them to do their work. It might not seem like much when there's just a millisecond of difference, but when every single transaction your computer makes with your hard drives is affected in the same way, and there can be hundreds or thousands of transactions a second, it adds up in a hurry. With hard drive access speed being the most notorious bottleneck in system performance, screwing with that makes everything seem slower.
For the record, my computer really isn't anything to sneeze at. Core2Duo E8400 with 2GB of DDR2 RAM. But Windows 7 boots to desktop in about 10 seconds.
