I am trying to figure out if it would be worth adding a video card to an old computer. I want to be able to play recent titles (ex. Battlefield 3) at low to medium detail settings and no AA at modest resolutions (1680x1050) but don't want to spend more than $75 on the card.
Is there any card that I can buy which will let me do this. I am willing to buy a used card from anywhere (ex. craigslist, ebay). I was looking at a Radeon 3870 for $35 or a Radeon 4850 for $50.
Here are the system specs:
Intel Core 2 Duo 2.13GHz
2GB RAM (Upgradable to 4GB)
280W PSU (No PCIe connectors)
250 GB Hard Drive
Your goals are unrealistic considering your budget. Buying used cards is a complete crap shoot. In my experience it's usually not worthwhile since there's a good chance the card will be DOA. I've actually purchased a card that was "tested and working" and when it arrived it clearly had toasted/leaking capacitors (which I would have seen had I looked closely at the listing photos). I've had similar experiences with two other cards.
If you're certain that the 4850 is in good working order it's not a bad deal at all, but it does require a single 6 pin PCIe power connection. If I were you though I'd up my budget a big and just get a brand new AMD Radeon HD 6670 which is the best card you can get without using a 6-pin power connector and it offers performance nearly equivalent to the 4850 you list.
EDIT: Asus HD 6670 with GDDR5 memory that's in your budget....