I would have to disagree with all the ECS K7S5A Pro bashing...I have one, and the system (I'm typing on it now- its a 1700+ with 512 megs of ram) is rock solid and there has never been a problem with it. Granted, its setup exactly with the same mind that you are using it for (its my parents pc, and they use it to type up their documents, etc. and my little 9-10 year old brother plays the occaisonal game on it) and its preforming well.
Furthermore, I disagree with many people who say that you need pc2700 or pc3200 ram. The next time you are seois about upgrading your system (Especially since you are still with a 400mhz Celeron this entire time), the ram you will have will be useless and you'll end up buying newer ram anyway.
IMO I would suggest two things:
-512 megs of ram- If you are going to be using WinXP, and even for just word processing/everyday office usage tasks, I have noticed that Windows runs faster with the extra ram. I'm not thinking in terms of game usage, but more like laucnhing applications that have been stored on ram (something that MEchBgon has mentioned before)
-Getting an IGP Motherboard. With your setup you will be runninat 100 dollars for the Mobo/Vid Card, and you can get this
http://www.newegg.com/app/ViewProductDesc.asp?description=13-138-234&catalog=22&depa=0 for 66 dollars from newegg (seems to have great ratings too), which is just as cheap as your current motherboard selection without forking over the extra cash. Others may say to get a better video card, etc....HOWEVER, if all he will be doing in playing the Sims and games of the sort, I don't think that you should be making a good videocard a priority. And if in the future you deem it necessary to get a better card...then you hvae a AGP slot
edit:
Oh- if you can grab those Frys deals (if you have one near you) I highly suggest you go for them

Last november I got a 2200+ and the ecs mobo (i ook the 2200+ b/c I doubt my parents can tell the difference

) for 49.99