People who blithely recommend 120 or 130 (or 180) watt video cards to people with stock computers, without factoring in power supply requirements, are silly or poorly informed.
An Inspiron 530 with a quad core includes a pretty decent 350W power supply, rated for 25A (300W combined) on the +12V rails. It's not bad. It wouldn't quite meet the power requirements of a GeForce 9800GT or Radeon 4850, but there are several good video cards that would work well.
A Radeon 4830 is probably the fastest video card for which a power supply with 25A on the +12V rails would be sufficient and reliable.
SAPPHIRE 100265HDMI, Radeon 4830 512MB, $85,
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102822
A GeForce 9600GT is also fast, and draws even less power (60 watts under load),
GIGABYTE GV-NX96T512H, GeForce 9600GT 512MB, $90,
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125099
This Radeon 4670 GDDR4 is a good home theater card (quiet fan, HDMI output), but it's also fast in games (drawing about 70 watts),
SAPPHIRE 100255DDR4, Radeon 4670 512MB 128-bit GDDR4,
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102818
(Those are in order of gaming performance. The 4830 is fastest, followed by the 9600GT. Radeon 4670 is still fast enough for most current games, though you may need to use medium quality settings for high-end games like Crysis.)
(reply)
Upgrading the power supply would cost at least $60-$75. So, it's not worth upgrading the power supply just to run a GeForce 9800GT or Radeon 4850, rather than a GeForce 9600GT or Radeon 4830. (Realistically, those lower wattage cards offer performance that's pretty comparable to their siblings. In some games, you may need to turn the quality settings down one notch, to make them run the same.)
If you are going to spring for a power supply, then you may as well go for an even higher-end video card, like a Radeon 4870 1GB, or even a GeForce GTX 260 216 core. (So long as you upgrade to a large enough power supply, you can run any video card you want.)
Examples,
Antec Earthwatts EA500, 500 watt (34A +12V), $80,
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371007
PC Power & Cooling Silencer PPCS500, 500 watt (35A +12V), $75,
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703015
SAPPHIRE 100259-1GL, Radeon 4870 1GB, $190,
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102801
EVGA 896-P3-1255-AR, GeForce GTX 260 Core 216 896MB, $180,
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130434
When upgrading a video card, one important consideration to keep in mind is the resolution you plan to run. If you have a 19" monitor that runs 1280 x 1024, and you plan to keep it a long time, then you don't need the ultimate high-end video card to generate good frame rates in games. Most decent video cards can generate playable frame rates at 1280 x 1024.
On the other hand, if you have a large, wide-screen monitor running 1920 x 1200, or plan to buy one soon, then you almost can't have too much graphics power. You should err on the side of buying a higher-end video card than you think you need (or, for a serious gamer, even think about running dual video cards, SLI or Crossfire).
(That's why I think gamers should avoid going past a 1680 x 1050 on the monitors they select, unless they're also willing to spend substantial money for the video card and power supply upgrades they will then need.)
(reply 2)
The wattage figure is actually kind of a poor indicator of power supply performance. Manufacturers of inexpensive or generic power supplies often inflate that wattage figure. (I've seen "500 watt" power supplies with only 17A on the +12V rail, and "425 watt" power supplies with only 13A on the +12V rail.)
A better indicator of power supply performance is the amperage rating on the +12V rail(s).
So, for comparison, for a GeForce 9600GT, manufacturers often recommend a power supply with a minimum of 22A on the +12V rail(s). For a Radeon 4850, manufacturers often recommend power supplies with 26A on the +12V rail(s). (For convenience, they conflate the power supply recommendation for the 4830 with that of the 4850, though in reality the 4830 draws some 30 watts less than a 4850.)
So, the fact your current power supply supports 300 watts combined (25A) on the +12V rails makes it look pretty good. Your system isn't likely to draw more than about 230 watts total under load (with the CPU drawing around 95 watts, and with a Radeon 4830 drawing 80-85 watts).
What I don't like seeing is when people replace a decent 350 or 400 watt factory power supply, with a really cheap power supply that claims to be "550 watts," because they're convinced the high wattage number guarantees the new power supply will be better than the one they replaced. Then they find out the new power supply either doesn't work right, or fails after two months. (The lesson being, the reputation of the brand, and the amperage rating on the +12V rails, are often better indicators of power supply quality than just the wattage claims on the label.)