The Athlon1700XP architecture is really old. The min req of a 1.8ghz probably reflects that of the new architecture Conroes and Yonahs , like my Celeron M or modern day Centrinos (conroes). New architecture chips are much more powerful then the old arhitecture chips. Your chips will struggle
For a quick fix...you could borrow spare parts and second hand stuff to upgrade that PC for less then 200$ probably..
for your AGP slot - simply search around for a second hand/old 9600pro , 9800pro 6600gt , X600pro . Im saying this because you probably have an old PSU you dont want to overload with the best AGPs out there - the X1950pro and 7800GS. If your psu is 400W or better , then maybe grab a X800 or a X850 , 6800 or a 7600 , or grab the best agp cards mentioned
for your

athlon slot. Upgrade your Athlon1700XP to a Athlon 3200XP Barton Core. This Athlon single core rocks as its architecture is much better then athlons under it.
For your memory just pile 2 gigs of cheap FSB400 generic .. its cheap.
With a Rig - Athlon3200XP , X850XT and 2 gig of of cheap 400FSB ram , you can play Supreme Commander 2v2 on 10km maps no problem , and sins will play like a dream.