Hmm, this is probably a dumb question but could I connect the NAS to my iMac via a CAT5 cable thus getting Gigabit speeds and still keep my wireless connection to my router? I would access to both at the same time pretty much all the time so wouldn't be able to turn Airport on and off.
That sounds feasible, but you might have issues accessing the internet, not sure how to tell your mac which route to take to the internet (default gateway and all that lark), in theory you could use a cable between the imac and the time capsule, and link the time capsule via a wireless bridge to your exisiting wireless thus your imac getting all network and NAS traffic via the ethernet port.
Shouldn't have any problem accessing the net, it'll be accessing the NAS that becomes the problem. The iMac will be given all it's network settings via the wireless router as per usual via DHCP, but the NAS won't be able to see the router and won't get any settings - I don't know how your NAS is set up but I assume it's via DHCP.
You need to hard code the IP settings to the NAS first. I don't know anything about it, but assume it must have some kind of options to configure network settings.
Give the NAS settings so it doesn't clash with anything already on your network and plug it into your iMac (you may need a crossover cable - depends on the NAS).
If the router is 192.168.0.1 and the Mac is 192.168.0.2 etc. then keep a few addresses clear in case you have anything else attached or decide to add anything. Use something like 192.168.0.20 with a subnet mask 255.255.255.0.
I don't know much about Macs and networking, but if wanted the NAS visible to the rest of the network then you can in theory bridge the ethernet and wireless connections. There may be some Mac people on here that know how to do that and then in theory you could leave the NAS set up to get it's settings via DHCP.
Bridging isn't something I've touched for a very, very long time seeing as switches are so cheap, and I've never tried bridging using wireless so I might just be talking out of my bum.
But still, regardless of bridging, you shouldn't have any problem hooking up you NAS via ethernet as long as you can diddle with it's network settings.