ISDN is a very reliable connection that can be an 'almost always on' connection like DSL or Cable. Ping speeds are generally pretty good for online gaming, and your download speeds will never exceed 15k/s unless it's an uncompressed file (.txt for example). Most of the time you will average 12k/s up or down.
If DSL is available in your area, get that instead, you'll pay the same for DSL vs. ISDN and the DSL will be faster downstream, and possibly upstream as well if you pay for it.
ISDN cost me about $60/month when my roommate and I had it, and he worked for Internet America so the account cost us nothing. I assume $20 would be a normal monthly fee for your account. Then if you want static IPs you will probably pay $5/month per IP, if you have more than one machine.
More than likely after all is said and done, minimum would be $75 per month, but hell, anything is better than dialup. At least on ISDN your speeds are almost always constant.
dm