I've used this
http://www.newegg.com/Product/Produc...82E16815150161 Siig card several times with great results - but it's only PCIe 1x. They also make a nice 4-port bay hub which goes with it. If you can find an ASUS "U3S6" card, that'd be the best. Cheaper, PCIe x4, and 2 internal SATA 6G ports... I used to order it from NewEgg under motherboard accessories.
Daimon
EDIT: IMO, the PCIe 1x cards make for an easier upgrade, as it's likely you'll have a free slot (unless you're a couch-potato with multiple TV tuners). PCIe x1 2.0 will still give you 5Gbps.