Do you really need USB 2.0? Most 2.0 externals will run in USB 1.1 slots you already have.
The only upgrade path toward USB 2 is adding a PCI card.
Something like: Rosewill NEC 4+1 Port USB2.0 PCI CARD
Note that USB 3.0 may be out soon - here is a statement about it:
The USB 3.0 specification guarantees an increased performance of 10 times compared to the capabilities of the current USB 2.0, providing a bandwidth of 600 MB/s second. Devices employing USB 3.0 specifications are planned to be available to consumers in 2009 or 2010 and would be backwards compatible with USB 2.0 and USB 1.1.
And a later update:
Yes you are right, no fixed release date for USB 3.0 . According to wikipedia The USB 3.0 specification was released on November 12, 2008 by the USB 3.0 Promoter Group. On September 18, 2007, Pat Gelsinger demonstrated USB 3.0 at the Intel Developer Forum. This move effectively opened the spec to hardware developers for implementation in future products. The first USB 3.0 ports were featured on the Asus P6X58 motherboard; however this board was canceled before production for unspecified reasons. USB-IF has scheduled a "Superspeed" conference for mid-November, which suggests optimism that the spec will be final by then. But You can now buy the USB 3.0 cable from Buffalo. It is 10-times faster than the conventional USB2.0 transfer speed. The cable is available in three models according to its length.