I'm going to try and answer this to the best of my knowledge.
"I understand Steam is PC based not mac, but crossover games supports it"
Correction, it's Windows based, despite what Apple advertises, a Mac is just an Apple branded PC, and a non-Mac PC can have other operating systems installed such as Linux (hence the Crossover Linux version). I'm sure if you installed Windows on a Mac, you would be able to run it just fine. So, normally a Windows based program would only run on Windows (obviously), but Crossover/Wine gives Linux/Mac users the ability to run a Windows based program natively on their own system. That doesn't really answer your question, but I thought you might like a bit of information, moving on...
"Does this mean, that the game Lineage II, also a PC game, not currently supported by Crossover games, can be played via steam and would be supported in that regard?"
Crossover Games officially supports the steam GUI, which means you can run Steam, and browse their store etc. but the games you install are not part of Steam. Steam will just launch the game for you (just like clicking the .exe file manually), but once the game is running it has nothing to do with Steam, it will be the same game if you ran it without Steam. So, if it doesn't work without Steam, it won't work with it. On the compatibility database it says known not to work although the highest version on the list of tests is 7.1.1, so it may be worth trying it on 7.2, but most likely it's not going to work at this time.
"ANd wouldnt it be a little slow playing a PC game via steam which is via crossover before it ever runs on the mac? Like interface overload? How much G would I need? I only have three and just playing Guild wars via Crossover is sometimes little slow (acceptable to be sure) but via crossover via steam?"
If a game runs properly on Crossover, in my experience the game will actually run better than on Windows. I usually see faster frame-rate and faster loading screens (mainly based off playing WoW (may also only be true when a game is run using OpenGL rather than DirectX)), my guess is that Linux uses less resources than Windows and so there are more resources for the game, and a more efficient filesystem results in faster loading. I've got no idea what G's are, G-force? Ghz of your processor? GB's of your RAM? GB's of space on your harddrive?
If you used Steam, as I said earlier it only launches your game, so other than using a little RAM, like any other programs you have running on your computer, it shouldn't effect the performance of the game your running.
Hope that helped.
