I would install Workbench on the PCMCIA card. 4GB is MORE THAN PLENTY.
You could do this using WinUAE. Then you would have to transfer the bootdisk to floppy with EasyADF. Format your card as FAT32 and mount it in WinUAE as a FOLDER device, you could then install workbench on it using the ADF files you have (yes, use the Install disk)
I uploaded a bootdisk image to The Zone (CFbootdiskA600v2.adf) for you to hold on to. The disk mounts the PCMCIA CF device, then assigns all system paths to that drive. This bootdisc assumes you have succesfully installed a Workbench into your CF card previously. There are other versions of similar bootdisks that mount a virtual hard drive file created in WinUAE as a Workbench partition. You can use that method too, I use this because I find it more convenient.
I don't know how EasyADF works but if it has to have certain files in the CF card keep in mind they might get overwritten when you install Workbench to it.
The advantage f doing this is that you don't have to spend an extra dime. I wouldn't buy the 2.05 ROM, makes no sense. Just keep what you have and use the PCMCIA port as your hard drive. It's not any slower than the internal hard drive, at least not in the use I have given it (I load my WHDLoad games off it instead of putting them in my internal CF hard drive). Also the PCMCIA port is pretty useless except for those expensive SRAM cards I wouldn't get (I would get an ACA620 accelerator instead if I did want to add Fast RAm to an A600).