Yeah, the first thing that jumped out at me on your list was the 15 inch monitor. I would spend some dough on that and keep in mind that a monitor is one of those upgrades that you take from computer to computer is a really not obsolete until it dies. A decent 19 incher can be had for under $400. By the time you yearn for a new monitor, the flat panels will be up to snuff in terms of price and quality. Your CPU is not that bad and if you can crank it to 558, go for it. 96MB of RAM is OK and you can get by. I don't know if I would add 128MB like prodigy says, but at least top it off with 128. You probably have a 64 and a 32 stick. Hawk the 32 and buy another 64 stick. Too bad you have a V3 3500 and not a V3 3000 as I would also say to just hang on to the V3 3000, but the V3 3500 is a notorious P.O.S.. The GTS would be overkill if you stayed with the PIII 450, so just get a GF1 DDR and you will be good to go. Here is the $$$ breakdown:
19 Monitor(Samsung Synchmaster is decent and a good buy) $375
64MB PC100 - $50 (tops, net of hawking the 32MB stick)
DDR - $170 (I just sold mine on fleabay for $177
What the above will get you is an awesome improvement to your current gaming experience. I assume you are a gamer.
LJ