Don't put two drives on the same channel and expect double performance... with IDE two drives in Raid 0 on the same channel will not yield much if any increase in performance to a single drive. If you are going to do RAID, you need to invest in another two IDE cables and put 1 drive per channel. Since you are using 40g drives that means that they are probably old, which also means that they may fail within the next year or two, therefore you probably should put them in a Raid 1 array, which has roughly the same write speed as a single drive but you also get fault tolerance (one drive can drop out and your server will still function) and a bit faster read speed.
Also, software raid has a lot more overhead than hardware raid and I don't think it does automatic rebuilding or hot-swapping... so you might be better off selling both/all of your computers and investing in a nice new machine with SATA Raid and a dual-core processor. You could even keep the ram you have if you went with a socket 939 based system. Otherwise, the backup system is a good idea, pump one of them full of ram with a Raid 1 array, the backup would have only 512 ram but if your main went down you could pull ram from it
... I would also keep the backup system up-to-date with the primary so that you don't have a lot of work to do to get it online.
Good Luck