Ok, first, sorry for screwing up the thread width, I'll try and resize the pic tonight.
Brian, the Vader bust (new) is not yet out and is on the right side in this picture. There are undoubtedly other, better pictures since this is from C3, but at least I know this one won't go down in the forseeable future.

Are each of the mini busts basically instant sell outs (as in, something I should preorder), are there any that can still be found now (for reasonable, close to retail prices)?
In general, unless it's an exclusive I would say no, they don't sell out instantly. Issue size (1000, 2500, 4000, etc.) is obviously a factor as well, though more so it will depend on the general popularity of the character. As for sell outs, well common sense will dictate that. Vader should be obvious that it will sell very quickly. It is Vader and a nice bust as well, it will go fast. I will preorder that, but not anything else coming out in the near future, though Yoda may fall into that category for me as well.
MSRP is generally $50 on these as I understand them. I've yet to pay that for anything other than an exclusive. Even my stormtrooper I only paid $35 for on ebay. As Nick has mentioned, if you have a local store around you may be able to beat that $50 price tag, even on a preorder, something I cannot do based on my location.
I saw in the previous pages you guys were mentioning a new Vader, which would be a cool one to get as well since the 1st version is a small fortune. When is this one out (or is it already)?
I don't know when it is out personally and I haven't seen any announcements, though I expect it will pop up soon as to when it will be available. Some of the more experienced GG collectors may well know. As mentioned above, I will preorder this to avoid the risk of losing out on it. See above for the pic.
Also, the "OT" Yoda mentioned a page or two back, when is that expected to be released?
I think it is this fall though I've not seen an official date. As above, I may preorder Yoda simply because of the popularity of the character. I don't know if the OT mentality will be strong enough to make it as popular as the Yoda with lightsaber/PT version, but I confess to being nervous about getting this one. As with the Vader, I missed out on the first and really want to have a Yoda in my collection.
Also, as a general question, do you think these busts are "worth" the $45ish pricetag?
If you've not seen these in person, they're smaller than you're expecting probably. Once you get past that, yeah, they're worth that price tag, though I couldn't be a completeist in them either, just too much cash overall I think. They're very well sculpted, nicely painted and detailed and they are relatively limited so a nice, higher end collectible than Hasbro stuff, though not a Master Replica or Attakus high end deal.
You should still be able to find C-3PO for under $50 though he has been out for a while, so at some point the liquid stock will be gone and his value will rise to some degree. He is a nice piece and I think a number of people here have it, though I do not. I'm mostly an Imperial kind of guy so I've not nabbed too many outside that realm. That's not to say they aren't great. Bossk and the Gammorrean Guard are spectacular pieces, they just don't fit what I'm currently after in this line.