I have AT&T network. If i want to buy the Sidekick LX from T-Mobile, can I?...

...Do I have 2 pay more? Can i do it? PLease help me!!! I really want this phone!! At&T have terrible phones!!!!!!!

no, verizon is the only open network that allows you to have other phones under their plans and networks.

Did you mean you want to use your AT&T network on your T-Mobile network based sidekick phone?
Yes means,you can do it.After buying your new cell phone,you will need to unlock your T-Mobile network lock on your Sidekick LX phone to use AT&T network.
You can unlock your phone by network unlock code.You can get an unlock code from your service provider or else try to buy an unlock code from any one of the online unlocking service provider like Unlocking4U.com Here you can unlock your phone at low cost.I hope it will be very useful to you.