FX
fx card that mobo will take would be
1950
that would be what i would say would max out your fx
note: I still having the card for more than 5 months and I have to say that the card still going strong, I was able to max Bioshock and still surprised by the performance this most powerful AGP card, the X1950PRO will have to run at a core clock of 835MHz to have the same performance as the XT since both have the same RAM bandwidth.
memory
the mobo will take up to 4gig of PC3200 but unless your on a 64os windows will only page 3gig
so maybe up the ram
CPU
as other have said grab a new CPU
as of software tweaking use XPlite pro gut everything you don't use and then you might get a little more free resources