Standby for an avalanche of go here ... go there ....
Countless places to be honest. Use Google to get a feel for prices etc. Always (given Charlie Chan's shop in Katmandu etc

) cheaper to buy on line. A minus point here tho, its easy to fall prey to buying something less than ideal in terms of the inevitable price / preformance compromise. Usually more dosh
Suggest you get someone locally - friend, collegue, workplace IT guy to have a look at what you have, so you get a sensible suggestion on a compromise. Particularly memory, its easy to say that all you need is .... whatever ... but there are varying quality memory chips (bang for your buck).
But .... dont think you will succeeed there with anything sensible
You beastie is under spec, it would not surprise me to find you get to the conclusion that a new box is needed, or cheaper/more economical than an upgrade. In the latter case, remember you dont
have to buy new screens, existing CRTs will do fine,dont pay for a flash LCD unless you want to, or really need to. Many suppliers will sell a box without a screen.
Its not a "throw it away" PC, it is fine for standard everyday things in life, and will be so for a few years yet. Its problem is that its not built for Graphics intensive applications / games because it lacks an AGP slot (AGP is a standard that "accelarates" the way graphics intensive programs work - latest successor to AGP standard is known as PCI-e). Its this latter bit that is your real problem, there is no real way to cope with the graphics intensive 3D that games such as Civ IV throw at it. It understands them, but it cannot process them fast enough with the results you see.
So, yes we can tweek as above, and maybe other things some may come up with that had the same problem, however in the medium & long term, spending too much on upgrade will be fools paradise - even expanding memory would do little as such without at least AGP standard graphics, and your PC board does not have an AGP bus, therefore no AGP card can be fitted.
Sorry kinda gloomy ....

But the time has come to get a new beastie ....
Regards
Zy