http://www.newegg.com/Product/Product.aspx?Item=N82E16814161319&cm_re=HD_5670-_-14-161-319-_-Product
Check out the HD 5670. Its $95 with free shipping and its a drop in replacement for your video card. It has significantly more performance and it'll keep you going for a while longer. That'd be a pretty good short term upgrade.
If you want a more 'serious' upgrade you'll need to change your CPU, memory and motherboard so:
2GB DDR2 RAM $36
http://www.newegg.com/Product/Product.aspx?Item=N82E16820134492
AM2+ motherboard + Athlon II Tri core 2.9 Ghz
$135
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.334872
So the total for a system overhaul is: $266, $302 if you want 4GB ram + shipping/taxes.
This system would be especially good for strategy games as they tend to like more cores in general.
Just one last thing, could you open up your system and tell us what kind of power supply you have. Brand + model number is pretty fine along with the rated wattage if you can see it.
Essay Pro