You should be able to find instructions for how to install a video card yourself online.

I post sometimes on the MacRumours forum and people there seem to know a bit about DIY stuff. There may even be links on the website to doing that - they have comprehensive instructions on installing your own RAM and upgrading your hard drive etc.

It will make a difference precisely what model it is... But if you know that then google should be able to help you. Some things are easier on some models than others.

Wikipedia can also be quite good with things like that...