1) If you only have 32-bit Windows don't get more than a 1GB video card. Windows 32-bit can only address 4GB in total normally and that includes the System RAM and Video RAM. So if you got a 2GB video card you could only access LESS THAN 2GB of your System RAM regardless of how much is installed.
2) We could help better if we knew:
b) RAM (amount mainly)
d) Windows version
e) BUDGET for graphics card or Power Supply AND graphics card?
f) current Power Supply
3) Confirm your CASE and Motherboard can fit a new graphics card.
After the cheap Corsair CX430 blew up on me, I spent a long time researching. I came to the conclusion that the Builder Series for Corsair is something to be avoided. I have a Corsair TX750 and it's been fine for many years, but you can't judge one product by a company's overall reputation (and vice versa).
The Antec I list has a 6-pin connection. (It also has 2x4-pins which can be used for a CPU 8-pin).
This partially boils down to BUDGET. The Antec I list may "only" be 450Watts but it should handle up to an HD7870 (possibly higher but I wouldn't risk it; overclocking also affects results as well).
AMD cards currently have a really incredible GAMES package. It's important to note that while an HD7770 is cheaper, you get MORE GAMES with an HD7850. And, while the HD7770's I looked at came with the main Far Cry 3 game, I'm not certain how well that game would run on FC3 (it's the most demanding game I own, and it's quite demanding on LOW quality as well.)
a) good cooling solution
b) good reviews (no warnings of "too noisy" or too many "died" or "artifacts on screen")
c) BUDGET (*accounting for game prices)
d) MIR (**Mail In Rebates) - The DATES often make it difficult or even IMPOSSIBLE to use. The MIR must be MAILED on or before the date shown. Products ordered on a Friday tend to arrive LATER than products on a Monday (weekend).
**Investigate if you can download and PRINT the MIR and send it away BEFORE your product arrives (your e-mail may have sufficient information to do so.)
e) card must FIT INSIDE THE CASE (check LENGTH)
*The 2.5 games have been priced at roughly $110 but I always wait and buy on sale. Tomb Raider was $25 (still is?) at Steam and Bioshock Infinite will be $30/$40 at some point. FC3 Blood Dragon should drop to $10. So for me, I'd call this $70 of value. Still, it's pretty impressive.
I double-checked the Asus HD7850 to confirm it uses only 1x6-pin. It does.
The GAMES offered by AMD make the decision of which card to purchase harder. I decided not to recommend NVidia for this reason as well. Rather than simply recommend one or two cards, I thought it best to state the REASONS behind this so he can decide for himself easier, especially if these cards aren't available for him.
The HD7770 still requires a 6-pin power cable. I'm unsure if a suitable adapter would work (he has no 6-pin). An HD7750 might work without an adapter but this will obviously limit the gaming potential of his PC (there are plenty of great games that don't require a demanding graphics card though but most modern games do.)
Again, though I recommend an HD7850 2GB as a POSSIBILITY and believe it will be compatible with the $40 Power Supply I link, the purchase decision is multi-factorial. Primarily it is the BUDGET (compensating for VALUE he puts on the games.)
Without knowing what types of games are to be played, I can only really recommend getting the BEST he can for his budget of the choices presented. So, if he can afford it, that's the Asus HD7850 2GB card (and suitable Power Supply).