I believe the biggest IDE drive is 1TB but its probably overpriced for what you plan to use it for. I have a 250GB and its plenty of room. CoinOps6 is 60 GB leaving 190 GB for games, most games are 3GB or less, which is easily enough room for 65 games. Ive read people have used IDE to SATA adapters, but I wouldnt bother unless you cant find an IDE drive.
This is the drive I used, and is my reccomendation:
http://www.newegg.com/Product/Product.aspx?Item=9SIA5AD2982973 The only things to look for are IDE, 7200 would be slightly better than 5400, and a bigger cache is always better. RPMs wont make a huge difference but will make a difference. Upgrading your cable to UDMA (80 wire, 40 pin) will also be a marginal upgrade.
How is your box modded? Softmod, chip, or TSOP?
If its TSOP'ed or hardmodded its super easy to upgrade the HD. Just FTP your saves, programs, games, etc over to your PC. Install the new HD in the XBOX, run slayers disc to prep the new HD, then copy everything back over. If its softmodded, I hope you have a PC with IDE channels.