An upgrade may be a good idea, upgrading your video card would allow you to run in a higher resoloution, this would make the graphics sharper, it may also allow you to run with the multisampling options turned up. this however depends on the maximum resoloution of your monitor.
The other thing that you need to consider is whether any of the current cards would work in your system, things have moved on a little since you bought your system. when exactly did you buy it? If it uses onboard graphics it means you probably wont have an AGP slot because the onboard system will be hard-wired into it. PCI video cards are quite uncommon now.... as are AGP cards to be honest. currently the standard is PCI-Express, its a much faster interface than AGP and PCI. sticking a new sound card in your system should be quite simple, not much has changed there, they nearly all use PCI sockets to connect.
If you only have PCI or AGP socket available you could probably get an AGP card from an online store or a shop but if you need a PCI card your best bet is Ebay or something like that, I doubt many stores will still stock them. the final option would be a new motherboard and CPU, if you get one with an onboard graphics and sound you should be able to run the game no problem, it would cost you about 100 pounds (180 US$) for the pair. obviously onboard hardware isnt the best way to go. But its about twice that price to get a decent upgrade with seperate videocard and soundcard.
