It is not Windows 98 that is the problem. It is the motherboard and what the motherboard will allow... Not many motherboards designed for Windows 98 are designed to use AGP. There are several different versions of AGP, and you have to use a video graphics card that is designed for the board you have.
For the good ones, the cost is high... above $75... but there are a few in the $30 range... The trouble is, there is no market for them, so hardly any retailer stocks them.
First you tell us your motherboard.