Badfish,
Upgrading your ram will improve system performance but you won't see that as an increase in fps in games because the 5500 is the bottleneck in games.
You've got enough processor power to get everything out of a 9800 Pro/XT or 5900/50 Ultra.
The Athlons are sensitive to ram latency. To get the most out of them you should get CL2 or CL2.5, Non-ECC, Unbuffered ram. You mother board supports dual channel ram so invest in a matched pair (generally refered to as a dual channel kit) of 256 MB sticks . Ram by Corsair, Mushkin and OCZ is considered best (I sure there are some others but Crucial is not among them and buying direct from Crucial is not the least expensive way to get ram. Try newegg). Good ram is expensive, around $155 to $180 for 512MB. You'd get a 10 to 15% increase in overall system performance from upgrading to the best ram. Maybe half that by upgrading to value series PC3200 ram. You may see a difference of a few FPS in AH2. If you spent that kind of money on a video card, you'd see far greater improvement in AH2, 15 to 20 fps AND the ability to run it with most of the eye candy
Any 5 year old monitor will do at least 75Hz at 1024 x 768. If as you say you want to upgrade to a 21" CRT, the Samsung 1100DF is a very good monitor that goes for $459.