Both of these look like good choices, and EVGA is a highly recomended brand. So that brings you to $146 before the rebate, and $106 after the rebate. Next, you will definately need a new power supply. How about I suggest this one
. It will be above your recomended minimum power requirements, which I prefer to do just in case.
So that will bring you to $156 after rebate. So lets compare it to your current computer. You will have a dual-core processor, which is good for current games, and multitasking. Your video card will no longer be "high end", but it will have DirectX10. You will have 1 GB less RAM, which is running at a higher frequency. You will be losing 80 GB of HDD space. Finally you should have a stonger power supply, which will probably be more energy efficient too (don't know what current power supply you have).
If you are good at managing your disc space, then you could possibly disreguard the lack of disc space. However, the amount of space needed by games is not a small factor. Bioshock will use up 8 GB of disc space to install. Another thing is it wouldn't hurt to upgrade to 2 GB of matching RAM.
Whether or not you decide to spend the money, I hope I helped out some.