I'm sorry but in my opinion its not worth upgrading and you would be better with a new PC.
The reasons are click here that is your board. Now it might be possible to upgrade the processor but its unlikely to be a huge boost and its AGP for the graphics so you could fit a better card but whatever you buy would die with the PC as a newer PC would not be AGP but would be PCI Express and the two are competely different.
Now depending on what you do with it at present you might be able to extend its life a bit without going bonkers cost wise. Its using onboard graphics I think you say so you could spend say £50 and get a basic graphics card but this will not turn it into a gaming machine, it would play a lot more but not the very latest. You could also either add a second hard drive £30-40 or buy an external hard drive, and lastly you could, depending on what you have add more memory.
Any money spent on an internal hard drive or a graphics card or memory will die with the PC as its very unlikely they could be used on a new PC. And since its not really an option to upgrade the processor then theres no real point in going overboard with any upgrades.
Think about it for a while and then decide. An external hard drive as an example would be good as you could put all your music on it and then use it on a new PC. Memory is cheap, maybe £20 for 512mb and a fairly cheap graphics card would help a bit. But upgrading to play the latest games is not an option in my opinion. Sorry.