Best: RAID 10
Better: RAID 6
If you must: RAID 5 w/hot spare
Whatever you do, consider buying new disks. Older disks are more likely to be near their failure point and rebuilding RAID arrays is never fun.
It depends a lot on what your controller supports. Some older controllers support only RAID 0, 1, and 5. From your account of this as an older server, I'd suspect that at most it supports RAID 0, 1, 5, and 10. RAID 6 is still relatively new to the field.
RAID 0 is out because it increases your vulnerability to failure.
RAID 1 in three separate virtual disks would perform all right and give you solid fault tolerance, but dealing with the extra volumes will be annoying.
RAID 5 has been the old standby for a long time, and with disks your size would probably be all right. But it has some major problems, chief among which is a the likelihood of irrecoverable read errors that can hose your rebuild. Say a disk fails. No problem, your data is safe in the parity stripe across the others, right? Unless there's a read-error in that stripe on one of the disks - then that sector is irrecoverable. The effects of this can span from very minor (broken temporary file) to a failed array rebuild (reference). The chances of a read error go up with the number and size of the disks in your array. Another issue with RAID 5 is that drives often fail in groups - and you can only take 1 disk failure before you're hosed. With older disks like you're working with, you're asking for trouble. Finally, and this may not apply, but a lot of older cards have shit performance in RAID 5. Definitely test this before deploying.
RAID 6 fixes a lot of RAID 5 problems by using two parity stripes, so you lose the capacity of two disks. The advantage is that if you have a read error in one of your stripes, no problem, chances are that parity bit will be readable in the other. It can also sustain two disk failures so the likelihood of losing your whole array is much lower. It tends to be a bit slower than RAID 5 on the same controller, but the performance loss is more than worth it. This seems to be the current RAID sweet spot.
RAID 10 is the most expensive RAID implementation in common use (in terms of disk space), but this is a case of get what you pay for: a RAID 0 striped array mirrored onto another. It can sustain up to 50% disk failure as long as no mirrored pair fails. There is no parity calculation overhead so performance is excellent. By far my preferred RAID level right now.
There are more exotic RAID levels not widely supported: RAID 50 (striped RAID 5), RAID 60 (striped RAID 6), RAID 3 (byte-level parity), RAID 4 (block-level parity) and others. 3 and 4 have some subtle performance differences from 5. 50 and 60 are probably the way of the future.
In the end, I strongly recommend RAID 10. If space is a problem, buy bigger disks. Relative to the value of your data, doubling your disk size should be very inexpensive (this is not always true, so take advantage of it while you can).