Any PCIe card should be compatible, so it really comes down to:
1) the BUDGET
2) choosing the best card and Power Supply within this budget
Your choices are AMD or NVidia (HD7000 or GTX600 series) and I recommend you spend between $200 and $350 on the graphics card though again I don't know the budget.
The choice of card has two additional decisions:
1) AMD offers free game coupons, so some HD7950's have THREE games so if you were to purchase these games anyway you need to factor this into the cost (the card is thus cheaper than it appears).
2) Don't get the cheapest version (like an HD7870) as quality and fan noise are very important as well.
With that said, I'll give a couple examples but I'll need feedback.
1) HD7770:
http://us.ncix.com/products/?sku=73018&vpn=HD7770-DC-1GD5-V2&manufacture=ASUS&promoid=1302
*This cards only $110 and if your son games casually or plays games that aren't too demanding this is still a huge upgrade. It has the added bonus of likely working with your current PSU.
2)
http://us.ncix.com/products/?sku=69585&vpn=R7850%20Twin%20Frozr%202GD5%2FOC&manufacture=MSI/MicroStar&promoid=1302
($190 after MIR, with two free GAME COUPONS, Bioshock Infinite and the new Tomb Raider)
There are more expensive cards but I'll stop there and wait for feedback.
Game coupons:
Please note that STEAM (install if it's not already) frequently has some really amazing sales. You can find great reviews at GAMESPOT and other places. There are some amazing games that used to be $50 that can be found for $5 or $10 now or on sale at other times.
Older games:
- are generally PATCHED better and have fewer issues
- are generally CHEAPER (may even package the game plus DLC/expansion for little money)
POWER SUPPLY:
Your case appears to support the standard ATX size. Power Supplies can often be found on sale, but it can be very difficult to find the best deal. You need to ensure:
- the TOTAL POWER is sufficient for the system (i.e. 450W or more)
- the PSU can support your video card (In general, just see if it has enough 6-pin and/or 8-pin connectors for the card if required)
- is a reliable, quality brand (can read comments)
This is my quick recommend of Power Supply:
http://us.ncix.com/products/?sku=76035&vpn=CP-9020047-US&manufacture=Corsair&promoid=1302