I hate to just dive in and be the bearer of bad news, but you ask us what is slowing your gameplay down? Well to be honest, almost your whole system is holding you back, and holding you down!
Intel Pentium 4 @ 3.0 Gig's = This might be the only good part of the system, if it's (H/T) even better!
1 Gig of system RAM = At the very least you should have 2 Gig's of RAM! 4 Gig's would really do you here, which a lot of places sell 4 Gig packages for decent prices. Well scratch my bright idea, just grabbed a peek at your board on Google, 2 Gig max support, forget the 4 Gig thing!
The Nvidia GeForce 6200 = Crap, crap, crap and a little more crap! The 6200 wasn't really meant to be a gamer card. You've got yourself a low level budget card! The 6200 were pretty much just a half a step up from most on-board video adapters of it's day. Plus most 6200 from back in the day were PCI slot only cards, not PCI-e or AGP! Looking at the motherboard on Google the board does not have a PCI-e slot, only a AGP slot, might want to get a better video card that supports that format.
Question - What is your power supply rated? 250 watts, 300 watts, 450 watts, ect. ect. ect. what is it? Knowing how much electrical power we have to play with here, will be a determining factor as to what this system of yours can be upgraded to.
Lastly, only speaking for myself, I would have never in a million years thrown Windows 7 on such an old system! I would have stuck it out with Windows XP, no questions asked here, I would have kept Windows XP as my Operating System. People might think I'm odd or strange here, but defendant upon the age and format of the technology I use in a computer build is the deciding factor of what type of Operating System the computer gets. Like your Nvidia GeForce 6200, that came along at the tail end of the Windows 2000 Professional era and at the beginning of the Windows XP era. I would then think I could go two ways here, run Win 2000 or Win XP, because both O.S.'s are true to the date of your hardware's release.
Increase the RAM from 1 Gig to 2 Gig's
Leave your Intel Pentium 4 alone, it's fine, it's what I use!
If your keeping that motherboard, go back to XP!
Kick the Nvidia 6200 to the curb, find a new AGP styled card.
Please click on the provided hyperlink, it will launch you to a decent little card, which I suspect would be way better than that 6200 your playing with. And this might stir up ideas of even better video cards to play with!
http://www.newegg.com/Product/Product.aspx?Item=N82E16814187043
Updated Edit : I was just thinking that if you want the "whole splash" for just hundreds of dollars, than I would hold off buying anything to extensive here for your current system. I would maybe start researching "all in one" or "budget" systems, I know lot's of people who purchased "all in one's" and are having a pretty good time when using their systems. The provided hyperlink is what I'm talking about, you might want to save up a little bit more money, to be honest your not that far away from affording one of these.......
http://www.frys.com/product/6376051?site=sr:SEARCH:MAIN_RSLT_PG
maybe give systems like the one I directed you to some thought! Forgive me for telling you it might be better all together to buy a new system, I'm just trying to throw out some ideas here!
Hope all this helps.....