Hello Xavii, welcome to PCHF.
lol, it's been long long time since we ever used PCI for graphics, and you'll never
ever be able to run COD 4 on a PCI graphics card.. old tecnology. PCI these days is only used for things that don't really require the speed, such as modems. Are PCI slots the only thing you have in your PC, or do you have AGP?
Your right Dell as very poor at leaving your PC open for any improvements. I have always recommended Dell for their quality PC's and technical support, but this is their downfall. It's normally quite hard to upgrade a Dell and you'll probably need to upgrade most of that PC, Power Supply, Case as well as motherboard and maybe
RAM.
A Backbone PC maybe good idea, you can also purchase these from Dell (using their customisable menu), but you can also pick them up from places like Comet. All they really are is the basic PC without keyboard, mouse and monitor.
If your looking at building your own, it would completely depend on what you budget is, what you use your PC for, and if your going to use the parts from your Dell (make and model required)?