The issue is you got a GeForce x4xx processor. You shouldn't go below 6 in nVidia land. They're not really meant for games ... the majority of shader cores are actually missing. A 240, is pretty much, meant for a system that can do HD video and some 3D stuff.
I just bought a EVGA GeForce GTX460 SC 1GB and it rocks. It's got a recommended power supply of 450W and it runs pretty cool (L4D2 maxed all settings, 50% fan speed 41C max) Also, EVGA, lifetime warranties. OH, you want an EE card. (external exhaust). The card is $200 USD right now (about) and comes with a steam copy of Mafia 2.
There's also the 560Ti, which does perform better than the 460, but it's pretty similar, and uses more power (500W recommended) and is $50 more.
Don't waste your money on a 465.
Something you can do, to note, is check the power consumption ratings on all your devices and figure out how much peak power you system actually uses. Cross reference that with the power consumption reported with the card you want and you'll have a more accurate wattage rating than what the manufacture guesses will be in your system.http://www.newegg.com/Product/Product.a ... 6814130568
To note, it sounds like your motherboard has a PCI-E 2.0 slot, there's really no limit to the GPU you can put in there. I'm using that 460 with an AMD Athlon 64 3200+ processor. So, there's not really a limit on the CPU either. Considering you have a quad core processor ... you shouldn't worry about it so much. Your main concern, really, is power.