You can use any provider with data service, but you'll need a phone that has a cable that can connect to your computer. Since you have a 2100, you don't have bluetooth built in and so you can't use a phone that just has bluetooth.
Then again, you probably want to use a cable anyway. The newer EDGE network, which is 3x faster than GPRS, only operates over a cable. It's available on the Cingular Nokia 62xx phone. However, it's still slower than regular dial-up.
You're probably better off using the sprint PCS or Verizon data service, which offers near-broadband speed, and doesn't use your phone but uses a built-in card into your computer.
If youre in LA, they're piloting the 3G wireless network on AT&T. That network promises speeds upward of 3 mbps. You'll have to get the uber-expensive Motorola Asomething brick phone but if you need speed it's the way to go.
I am returning from asia and have a compaq presario 2100 and want to know what service provider and phone with accesories I should purchase in order to go online using dialup through the cell phone on the West Coast.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic