Well if you are looking for a PCIe card to do it, then you could use the
Areca ARC1220, but that is a $600 RAID card and you would only get the maximum throughput that a single PCIe lane (250MB/s) could give you (Although I doubt you could get anywhere near that much data from the drives and the card) But if you put off buying that second EVGA 7800GT that your rig profile says you are getting then you could get the full benefit that it would receive from all the lanes. It performed admirably, though on a PCIe 8x lane on
this review.
I doubt you want to shill out $600 for a card, so my next suggestion would be a motherboard that supports 6 SATA 3.0 Gb/s drives. Asus just came out with one this month called the ASUS M2N32-SLI Deluxe which can you read a review for
here. This can be bought for about $190 at newegg (OEM though). This would require an upgrade to AM2 though, so that might not be what you want either.
A 939 motherboard that uses the nicest of the 590 SLI chips I believe would support up to 6 SATA3.0 Gb/s drives, but I don?t know of one off hand.