I don't have my amd's 100% straight but I believe a athlon xp 2600+ or better sould drop in and they go up past 2.2ghz which would be a big step up from the duron (celeron equivalent for AMD's line for that mobo iirc) and the 3850 is a very good card (for the money).
TBH you could probably snag a Athlon XP 3000+ off ebay (assuming the board would recognize it) for around 35$ or so with a tiny bit of patience.
About the only bad thing about it is that its not a dual core processor which would make the multi tasking the OP is doing a bit smoother but we are talking about spending ~135+ shipping for the card and ~35$ for the processor (and itd still do fine for current games at lower resolutions).