With that chip it's quite likely limited to 100mhz memory and no more than perhaps 1 GHz Athlon - hardly worth the time and expense (and risk of getting a dud CPU off Ebay). Get more information about your system with this; click here
Even if you have or can fit 133mhz memory, your limit is likely a 1.4GHz Athlon - that would make a reasonable difference but the cost will be disproportionate.
There are still plenty of AGP cards around, but there are three types of AGP connector - check the results in PCWizard for your one.
You can fit a USB2 PCI card if you have spare slots;
Assuming you have Win98 Second Edition too, USB support was pretty flaky before that.