I have tried a number of options but the easiest I have found that had NO PROBLEMS WITH ANY ATTACHED DEVICE was the Edgeport USB to Serial boxes.
I have 4 of them and they don't need additional power beyond the USB connection and once you get the latest driver from Digi, they work flawlessly.
I had the Digi 8 port add on PCI board but it finally gave out when I went to XP-PRO. The new drivers didn't help. Still have that and one of the 4 port PCI boards from Byterunner and could never get it to work.
As I remember, the Digi board was around $400 when I bought it. Still have it and the Byterunner 4 port boards just in case anyone has a need for them. Will make you a GREAT deal. Not doing any good just sitting in a box in the closet.