first, read the overclocking sticky in the overclocking forum. It's not exactly your processor, but it gives you an idea of HOW to overclock. At best, you could get it to 1ghz me thinks...
I'm looking up your board here, and you're allowed up to 1.5 gb of ram. I think that would be too much for your processing capabilities, but 1gb might end up being about right.
You can upgrade your processor to a P3 instead of a celeron if you wanted. I know they made the P3's a little over 1ghz, so it might be an option. I know a 1.3ghz celeron exists. My friend has one in his brand new laptop.
You have a 4x AGP slot, so I'm going to recommend maybe going with an FX5500.
evga fx5500 128mb
That's about all the juice you can suck out of your setup! I hope things go well.