|You're getting hung up on framerates. If your game is playable & enjoyable & there's no lag, what difference does it make if the farme rates are 50FPS or 100FPS. Do you think your eyes can tell the difference? If you hadn't run FRAPS or whatever you used to check the FPS, I doubt you'd be concerned.|
I don't know how long ago you bought the GTS 450 but it sells for about $100 (after rebate) all over the place these days, including newegg. The GTX 460 sells for about $130 (after rebate).
Your eMachines motherboard is based on an old/outdated nForce4 chipset that was released over 5 yrs ago. The PCIe x16 slot is version 1.1 so it only supports 50% of the bandwidth of PCIe 2.0 or 25% of the bandwidth of PCIe 3.0, plus the board only supports 1000MHz HT, not to mention it has a weak CPU. I'm sure the card will work fine in it, but it's probably being bottlenecked.
Check this GTS 450 vs GTX 460 article: