I need to upgrade my GPU but I'm not sure if it's compatible! help!

[sakesuqs]

I'm currently using a NVIDIA GeForce GTX 275 running on an ASUS P6T SE Motherboard. I need an upgrade on my GPU because I recently bought a 27" monitor and the display isn't as clear as it should be, even after I've installed the driver. I am assuming it is because of the outdated GPU. I want to upgrade it, but I don't know if the newer ones are compatible with my current mobo or power supply (how do I even check which one I have?). Also, would I need more RAM to run the new GPU more smoothly? I only have 3GB but I'm planning on upgrading to 8GB in the near future. The only thing stopping me is I can't get my hands on a 64-bit Win7. Please help!

 

[sakesuqs]

Oh and my budget is ~$300

 

[sakesuqs]

I have an Antec 500w Power Supply