Will a ATI HD4850 graphics card work on my pc?

Yes your computer could run the ATI HD 4850,
But you will need to upgrade your power supply to a 500+ watt unit. (I dont know your computers PSU)

No you will not be able to run both cards at the same time.

Yes you will be able to play COD: MW2, but of course what resolution would you be playing at?

The most I'd recommend running under that configuration would be an HD 4670. Anything above that and your CPU will not be able to keep up with supplying it with information which will significantly degrade performance.

Is there a reason you need an HDMI cable? Are you running it on a TV or are you using a new monitor with that old system?

Call of Duty: Modern Warfare 2 Graphics Performance Benchmarks: AA/AF Disabled

Those are the frames per second with a Core i7 and much better components. Your CPU will limit either to about the same point, and your amount of memory will also cause a problem. I know you're thinking since it gets higher FPS than the HD 4670 it has to be better, but your setup will only allow for a certain amount of performance. Also, the HD 4850 will probably not be an option due to your presumably weak power supply which is a huge thing to take into consideration. Adding a GTX 295 or HD 5870 will also give about the same performance, it's just how it works. Spend less money and get the HD 4670 unless you plan on carrying the card over to a completely new configuration later. I might recommend spending the money you saved on another gigabyte of memory.

It will run COD6, albeit slowly unless you're running under 1680x1050.

SAPPHIRE 100295HDMI Radeon HD 4670 512MB 128-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Retail

Also, you can use both cards for multi-monitor support but they will not be able to be used in any type of CrossFire/SLi configuration. You will have to use the HD 4 series card for your primary tasks (gaming, main monitor, etc.) and the second GeForce card will have to be used just to render the second desktop.

I wouldn't buy a 4850 with the expectation of putting it in a future machine. It's overpowered for your current setup but it will be underpowered for a new gaming rig. They've already got several cards on the market that are better, and you'd probably want one that can support DirectX 11 so it doesn't go obsolete before the rest of your new system.

Honestly, as other people have said, you're not going to notice a hell of a lot of difference between the 4670 and the 4850 on your current machine, because the CPU and the RAM are the limiting factor.

I wouldn't buy that power supply, because it only has one 6-pin PCIe connection. If and when you build a new system, you'll want at least two. The really high-end video cards will often take two 6-pin connectors EACH. Sometimes an 8-pin connector. So there's really no point in buying that power supply, because you'll just end up buying another one later anyway.

ok. I've decided to simply not upgrade at all now, just going to wait till i've saved up enough then build a whole new pc. thanks for all your help anyway.