Sorry to be so blunt, but that system is so much underpowered that any investment is a waste of money. Better start saving for a new machine.
If you look at the Benchmark Results and click on the Rank column twice to reverse the sorting order, you will see all the bottom performers, for instance the E6700, at the bottom of the list and the E5700 is even worse.
I agree with Harm:
At this point you should save up for an entire new rig. As you can see from the PPBM5 results list, a system based on a Core 2 Quad CPU (unless overclocked heavily and/or equipped with a ton of RAM) cannot beat a system based on a Sandy Bridge dual-core i3-2100 CPU. Core 2 Duo CPUs are even slower than the Core 2 Quad example. And the Pentium E5700? Fuggedaboutit.
And even if you could upgrade the CPU and add a discrete GPU and additional disks, the Gateway is still saddled with a very limited motherboard that does not support RAID and lacks any overclocking provisions at all whatsoever.
In the end, you just have a system that (even after upgrading the RAM to its 8GB maximum and the fastest Nvidia CUDA GPU and adding two additional fast hard drives) is still much, much slower overall than even an i3-2100 system with only 4GB of RAM, let alone a fast i7 system with 16GB or more RAM. Thus, any upgrades that you make to that PC is an almost complete waste of money. (And as I discovered, every single one of the Core 2 Duo systems on the PPBM5 results list took more than 1,000 seconds to complete all of the PPBM5 benckmark tests with MPE turned on.) And even the higher 3GHz clock speed of the E5700 is not high enough to compensate for the deficiencies of the Pentium Dual-Core CPU (less L2 cache than the Core 2 Duo that the Pentium Dual-Core is based on), so that the E5700 is just as slow as, if not slower than, the E6700 systems near the bottom (lowest-ranked, or highest rank number) of the PPBM5 results list.