First, this is not possible. It is possible to use your iPhone's 3G or EDGE connection on your Mac via Bluetooth or the USB cable, but it is not possible to do the reverse.
Secondly, You seem to be a tiny bit confused about wifi, which is perfectly understandable. A wifi router doesn't limit the speed of your connection at all, unless you were to have an exceptionally fast connection, because, right now, your connection speed is most likely under 10MB/s. I know it SAYS it's 100MB/s, but that is just the speed of your home network (the speed at which data may pass between your computer and your current router, switch, hub or whatever you've got). The external speed (the speed at which data may pass between a server somewhere and your current router; in other words your "internet speed") is the limiting factor. Unless you've got a fibre optic cable going into your house, then your connection is going to be considerably less than 100MB/s. My own connection speed is 3.5-4.5MB/s, and it's a broadband internet connection; one of the fastest available in my area. If you'd like to find your own internet speed, go to this site http://www.speedtest.net/ . It will tell you the 'Ping' (how long it takes for a message to make a round trip from your computer to a server somewhere), your upload speed (the speed at which you may send data from your computer to a server), and your download speed (the speed at which you can receive data from a server). For most intents and purposes, the download speed is the only one that matters much.
Anyway, back to wifi routers. All a wifi router does, is "split" your internet connection, so to speak, into two streams; wireless for your iPhone and wired for your iMac. You will still be able to get the same speed, because it only splits the connection it's self; not the speed. In any case, you then have two internet connections. There is one more added complication. Not all wireless connections are the same speed. Most routers broadcast at two or more frequencies. The most common frequency, and the one used by ALL routers these days, is 802.11G, or just Wireless G for short. This one has a maximum speed of 54MB/s, which (although only half the speed of your ethernet connection) is fast enough to not limit you. Your iPhone can connect to a wireless g connection. So, this is the one you'll need. Some new devices and computers can connect to a Wireless N connection. It supports data transfer speeds of up to 300MB/s over the wireless connection. I've got both kinds of routers, and I don't really notice the difference.
So, just get a wireless G router, and you'll be set!