What's your total budget? It's difficult to make a reccomemndation w/o knowing your budget.
The FX-5900 is very expensive, too expensive for the CPU/Mobo budget you are considering. It's about $350 just for the card. You'd also likely need a new power supply to drive that card if you are using your old case.
There are lots of good cards in the $125 to $195 category, but again w/o your budget it's tough to help.
You don't have to buy WIN XP. If you do, you can keep your old CH analog gear, it will work, it just won't be programmable in XP. So you could dual boot to Win 98 or XP. Use XP 99% of the time, but when you need to reprogram your CH gear, boot in WIN 98.
Mixing analog and USB CH gear is not reccommended. It's all or nothing. Some folks may be lucky with the USB adaptor, but I wouldn't count on it.