"goodTalk" chat application, done in ExtJS. Criticism needed!

Hello everyone, after more than a month of simultaneously coding and learning extJS i have finally come to an appreciable milestone in my project, which was all about creating a real-time chat site. I am writing here to promote this project, and get further ideas.
Special thanks: ExtJS community (most notably Condor and Animal), superdit.com, extjs.fudini.net.

I have no professional experience in programming since i am a college student, so any and every advice and criticism, both constructive and destructive is appreciated, especially from seasoned programmers.
Both Javascript and PHP code are present. I will also post questions of my own regarding the project:

1) I use the same MySQL table both for registration, and operations in real-time done on the account. Should i use two tables instead?
2) The project is intended to be a real-time chat based on ajax, polling each 0.5 seconds for new messages, and each 4 seconds for buddy status (online/offline), but that's inherently very taxing to the server. Should i change it to a pseudo real-time chat, updating new messages each 10 seconds? Would that be catchy?
3) Doesn't the name 'goodTalk' raise copyright issues? Most notably with google's gTalk ?
4) What visible (or covert) security holes are in the project. (I implemented prepared statements wherever needed, but then again, i'm a noob)
5) Is that paypal donation button functional? (no, i'm not suggesting anything)
6) Please suggest some cool free hosting/domain sites, if you know any.
7) There still are bugs most probably, you might encounter them as soon as you try anything out, so...yeah.
8) Anything else on your mind, like the code inconsistencies you might find.

Thanks !

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(images.zip must be extracted in the 'images' subdirectory created by extracting the project)
EDIT: The SQL file contains a naming inconsistency - it creates the 'goodtalk' database, and then creates tables for 'chat'. Rename 'chat' to 'goodtalk', or the other way around.
EDIT: Added SQL file