If you have a memory slot that is "blown" and not functioning, you have a defective motherboard. Correct that then you can worry about performance tuning.
If you have a Celeron processor, the biggest performance gain will come from changing to a full Pentium 4.
Special software isn't required for overclocking, although some board manufacturers provide utilities for doing so, the memory timings can be adjusted manually in the BIOS setup of the motherboard if the motherboard supports it. Here's a
list of stability and performance testing utilities.