More RAM. You'd be surprised how quickly 128mb runs out. You should be able to find a 256mb PC133 module fairly cheaply if you search around. Just make sure the largest installed module is on the first bank ('Slot A0'
The hard drive, does it still have its original? A 5200rpm hdd from 2000 is going to be much slower and noisier than a decent 7200rpm liquid-bearing hdd from today. Hdd speed is very influential on launch and boot times, especially if you've only got 128mb of RAM and it's digging into the swap file. 80 to 160gb hdds are a good price now.
There're lots of people who want to upgrade their Slot A system's CPU, so the faster Slot A CPUs sell for exhorbant second-hand prices. But you may be able to buy a 750MHz Slot A CPU for not much more than what you could then sell your 500MHz Slot A CPU for, if you accept that hassel for a small increase in speed.
If you want to overclock, your motherboard might allow you to jumper the front side bus to 103 or 105MHz, which would be a 5% overclock.