For map design I would really recommend a big screen, if you have the space for it of course. It's just very helpful to see a large part of your map on screen without it being cluttered by all kinds of dialog boxes and panels and stuff (and most GIS/Cartography software packages seem to be very keen on those...)
Processor and ram I always recommend to try and max it out within your budget. If you want to stick with your Macbook Air and its specs are such that you can easily work with it then there's no need to do anything about that, but from what I gathered the Air's are a bit low on ram (the current model has 8 GB max if I read Apple's website correctly and I wouldn't want to do any big maps on that to be honest).
Storage space is another thing to look at, if you're working with large rasters you can easily rack up several gigabytes per project.
For input I personally use a Wacom tablet (instead of a mouse). Especially when it comes to fluid lines that's a big plus compared to a mouse (at least for me, I'm left-handed but mouse right-handed, but that always seems a bit unnatural to me...)
My current setup:
27" iMac as my prime workstation. It has the dual quadcore 4 Ghz i7 processor and 32 GB ram. That's what I have the Wacom on.
17" Windows laptop for mobile and backup use (plus for the handful of apps I use that don't run on Mac). It has a SSD for booting as well as a regular hard drive. 16 GB ram, expandable to 32. 3.2 Ghz i7 if I recall correctly.
A Synology NAS for storage (networked so I can access it from both machines), it's using 4 x 4 TB drives in a RAID configuration for a grand total of 8 TB (I'm not even close to filling that up but when I bought it last year the pricing of hard drives was such that any kind of lower capacity would have maybe saved me $50...)
I'm considering getting a 2nd screen for the iMac (or maybe one that I can switch between the iMac and the laptop).