While the wattage of your current power supply might suffice in most cases, the dual 12v rails in your power supply is the main concern to me with regards to your situation. The wattage should be ok for a card in the $150 price range, but the power split across multiple rails might leave the card without the power it actually needs, causing shut downs when the card is under stress. My take on it is this: you are pretty much going to need a new power supply if you do plan on doing higher-end gaming. Be absolutely sure to get a power supply with a solid single
12v rail if you want to start gaming with higher end cards. The reason is because high end video cards might want to utilize higher amps than what individual, separate rails can offer. A single rail of, say, 12v and 100 amps will be able to offer guaranteed power to the video card at all times because all of the amperage is there on the one rail, available at any time when components in the system need it. If you have multiple rails (which you do), then what's essentially happening is you're dividing up the available amperage across rails which means less available amps per connection.
I recently had to get a new power supply when I got a new video card and experienced the situation first hand. Though my power supply was rated 700watts, the computer would automatically shut down as soon as the system tried to stress the video card, due to the split 12v rails. I ended up having to replace it with another 700watt power supply - but this time with a single 12v rail - and it worked like a charm.
EDIT: I've customized a power supply search on Newegg (500-600w, single 12v rail, PCI-Express connectors). Lots of options, prices as low as $19.99 (although obviously I don't recommend going that cheap). Newegg's in-house brand "Rosewill" makes pretty solid power supplies, as does OCZ:
This post has been edited by HeartAttack: 25 May 2012 - 07:03 PM