If you upgrade anything, it should be your processor and mobo / memory. That e6750 probably bottlenecks the GTX260 at stock speeds to some extent. And not all games are pure GPU power for performance. A lot of games, along the RPG realm especially, use a lot of processor power as well. Here's a decent scaling example. I own a GTX260, and run an AMD Phenom II X4 Black edition (AM2+) at 3.65Ghz. At its stock CPU speed (3.0Ghz), my GTX260 didn't scale well on performance when overclocking the GPU. However, when I overclocked my CPU, I scaled very well in benchmarks and stability. Thus, my problem was my CPU bottlenecking my GPU.
Why not push the CPU further, as well as GPU?!?! My current power supply, an Antec 850 Quattro, and/or my DFI LanParty DK (1st gen) will not support the voltage, or more likely, amperage (on the power supply's part with 18 amps per rail, i.e. about 30 amps max with PCI-E power coming from 2 rails) needed to OC both at the same time. I currently play everything max on everything I own, including DA-Origins & Source games, very well on a 30" 1080p Sceptre Monitor. My card is backed down to stock voltages and clock, and my CPU is ramped up to get the most out of my GTX260 (a 192 core I might add). Hence, you need a new processor bud, cuzz your numbers probably won't go up any with a 5000 series ATI, assuming the lack of a Physx processor that the 260 is giving you, doesn't make your performance with an ATI card of any type go down.
Another way to look at it is that your 65nm CPU is trying to process input from a 65 or 55nm GPU that was made with newer technology and doesn't know when to quit. Try FAH, my stock GTX260 192 core pushes 6-7K PPD. I would wager a e6750 in at the 3-4K PPD range max. It just can't push it. $400 in mobo, CPU, and RAM will do you a lot more good than $500 in graphics card when you aren't even using the one you have's potential.
