The spec page for your machine says it came with 512MB of PC3200 DDR RAM. Find a sale on 2 x 1 GB sticks to boost it to at least 2 GB. If you're using the onboard video, more RAM will help, since it will free up the memory your machine dedicates to video. You can sell the 512 MB stick to help offset the cost.
The specs say the machine came with XP. Stay with it. Vista will slow it down. You can max out the RAM to 4 GB, but the 32 bit version of XP can only use is a little over 3 GB.
The machine has a PCI-e x16 slot. Adding a separate vid card will free up some memory and may offload other resources from the system. How much it helps will depend on the card you get and how well Gateway has implimented the PCI-e x16 spec.
From what I could find, the power supply for your machine is rated at 300 watts. If you want to add a more powerful vid card, consider getting a more powerful supply, as well.
Unless the CMOS shows options that allow you to set your cpu clock and multiplier, you won't be able to overclock it.