Since you want to play games, let’s assume you want to buy the graphics card NOW as its summer, and you have more free time.
The PC you've got has a very good processor. It will run top games quite well.
Your graphics card is poor, though. If you want to play newer games with high resolutions and decent settings, you need to get a better one.
You also MUST change ure power supply unit; 300 Watts are not enough to use the lowest-end cards, like an NVIDIA 9600 GT. get between 450 - 550 W, tht will b sufficient for almost anything u get.
So, we'll assume you want to play on the max resolution of ure monitor. ATI in general is cheaper than NVIDIA, but NVIDIA is, i think, a better vendor for very-high end gaming and they generally run with games slightly better (though many might disagree) but tht doesnt matter for you; u probably aren't going for a very-high end solution, like 2 or 3 graphics cards.
Im getting the impression ure budget-limited, so you will probably be better off going for ATI; but remember, it depends on what card level ure getting; sometimes NVIDIA offers something better at the same price, so b careful, shop around, use www.newegg.com, its great, and use this link for nvidia:-
http://www.nvidia.com/content/HelpMeChoose/fx2/HelpMeChoose.asp?lang=en-us
This is a great article, have a look at it, though the new month is here, they’ll probably come with a newer updated article soon:-
http://www.tomshardware.com/reviews/radeon-geforce-price,2323.html
Go to the conclusion and have a look at the table, it’s a magnificent reference for performance tiers.
Since you’re upgrading, I advise getting NOTHING BELOW a 9600 GSO/ HD 4670 level card, but it also depends on ure budget.
Then, you need to decide if you want a 512 MB for a 1 GB card; at 1680 x 1050 I strongly recommend a 1 GB.
Good luck. = )
P.S - If you tell us what games you want to play, at what resolution, at what graphics level settings, and especially what your budget is, we could advise you a lot better on what specific card to get. Tc.