Joe: The greatest potential (read: costly) pitfall to upgrading a system comes from having no experience or background to do it yourself. I sincerely recommend that you find someone, or a business who can provide you with some options to choose from and do the upgrade if you decide to have it done.
I only say this because if you're unsure where to start this, I don't think you will be able to successfully complete the upgrade you wish to do by yourself.
There are so many
undocumented hardware/BIOS compatibility problems that you may ultimately spend more money trying to fix your system than if you bought a new system.
And be very careful of the free advice here...
For example, AH2 doesn't use much video card memory and a 256MB video card is not necessarily better than a 128MB card. The speed of the card is related to the memory quality and not the memory capacity. If you're only hauling enough to fill 1/3 of a pick-up truck, a tractor trailer won't haul it better or faster. But, it will cost you more money.