Don't buy more memory unless you really need it. Few games require above 512MB, and even fewer programs really need more than that - especially on Windows 98.
My system only goes above 512MB when playing on the new very large maps in Forgotten Battles, or when playing UnReal2.
Aces High doesn't really need more than 256MB with Windows 98.
If you do video editing, CAD design, or something that requires a large amount of RAM, then it will be worth it to upgrade, or if you play games that want a lot of memory, otherwise you may as well save the cash and hold off for a while.
What type of memory does your system use? PC100/133 or one of the newer/more expensive types?
-SW