He was answering the question about flashing from a mac. I don't think there is a way to flash from the mac itself like you do from a windows PC. I was looking it up yesterday and didn't find anything to concrete on it.
You can flash from the internal storage drive of the phone though and that was what Mrobvious was telling you to do. The link below is to the thread about flashing, unlocking and so on. If you scroll down to the section about flashing from internal storage it gives you direction. If you search through your phone settings and find a button/icon called "usb to pc " you can set your phone to act as a usb drive. This only gives you access to the internal storage from whatever computer you have it connected to. Also when it is plugged into a computer and you have it setup this way the phone will not see the internal storage. So if you try and use and app that is on your storage drive and you have it plugged into a PC as a usb drive you will get an error.
There is also a selection to get it to ask you whenever you plug it into a computer. I use this as my work computer won't let me active sync my phone.
http://forum.ppcgeeks.com/showthread.php?t=36062
AS for the rom it is up to you. The major differences between the two are that 23541 the start and X button are at the bottom of the screen and 21895 they are at the top. There are some other differences under the hood but that is the one you notice the most and it comes down to personal preference. I personally use the 21895 rom build. I run a lot of programs that don't like the bottom button though so that made my choice pretty easy.