No it will not improve FPS. The GF210 series is a pathetic card at best. The 6600 has a much more powerful GPU on it compared to the GF210s. Yes, they have more RAM but they aren't even powerful enough to use more than 256MB so the 1024MB is a complete waste. Both of those cards will be money spent on performance reduced. The biggest problem I see there is your CPU. The 6600 can game decently but it will need to offload some of the graphics work in many games to a CPU that can handle it. The Pentium-4 does not classify as a CPU that can handle it. If you want to have a decent gaming system, your absolute minimum requirements are an AMD Athlon II X3 and an ATi Radeon HD 4670. That combination will play most games out today at a reasonable level. These little crappy changes that you are considering are a complete waste of time and money. The proper upgrade doesn't have to be expensive though. Here's what it will cost you:
AMD Athlon II X3 3.1GHz: $75
Foxconn M61PMV Motherboard: $40
XFX Radeon HD 4670 1GB: $50
Prices are from newegg.com and include shipping to the continental USA, add whatever taxes may be applicable but you're only looking at $165 because Pentium-4 3GHz CPUs are socket 775 CPUs which means you can re-use your RAM. Unfortunately, you cannot put faster Intel CPUs in our motherboard because Pentium-4 Socket 775 motherboards do not support the Conroe architecture for Core 2 or Pentium Dual-Core. They only support Prescott (unless you have some special motherboard that I've never heard of). If you do these 3 upgrades, you will be unable to recognise your computer because of how fast it will seem. hope this helps you!
"Avro Arrow" up here gave you good advice, or in my point of view, simple buy a Llano APU A8-3850 and a gigabyte motherboard GA-A75-D3H (rev. 1.0). With this type of system you will be able to play the newest games on medium to low game settings on all directx settings you set, without discrete gpu, and of course on a resolution lower than 1080p.