I'm new to the Amiga with a 1200 as my first machine. Strictly gaming. I don't mind buying the 55Mhz accelerator, but will it benefit me in any way over the 25 or 40?, ie, is the GUI going to be noticeably more responsive?
The 40 MHz ACA is very fast, and for the money it is an excellent buy and will give you a faster Workbench experience than the cheaper 020 cards. But if you're strictly gaming, then why do you ask about GUI?
If you don't mind buying second hand goods, and you have some cash to spare, then there are faster cards around such as the 68040 and 68060 Apollo and Blizzard cards.
Then again, if you're upgrading from a stock A1200 or one with just Fast RAM in the trapdoor, then even one of the cheaper 020 cards will be a significant boost!
My advice - get the 40 MHz 68030 card, then later down the road if you want even more power then perhaps the Vampire 1200 will be released by this time which will be faster than the fastest 68060 cards.