I am considering getting a Blackberry 8700 with Cellular One, and I have a few questions.
Can it access AOL Instant messanger?
And also can it go on really any website, such as myspace or different forums or mobile-only websites, like yahoo! or the weather channel?

Thank you!!

Yes, you can access AOL IM from your 8700.
I have used Ramble (free) and also tested YAK (not free). Both seem to login to AOL IM.

Wirelessly posted (BlackBerry7290/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)

Ramble really is the same as yak-on except yak has aim icons and u can sign on to msn and icq aswell as aim so if you use aim only I would suggest just go with ramble if you don't want to pay over 50 bucks for a really good im client also I have accessed myspace on my bb but its not the best on here though it does come up just like the computer would
Feel free to PM or IM me for any other questions

Don't use the OZ AIM client from RIM. It uses SMS messages and you'll have a hefty bill if you don't have an unlimited SMS messaging plan. You should download a third party app whether you pay a fee (Yak-on) or not (RambleIM).

The Oz client is actually via T-mobile, and yes uses SMS (at Tmob's request).