Comcast intially installed an older Motorola rental modem in my house. I bought the Zoom 5341 (Bestbuy $80) and my download speed went from 9.5 mbp to around 24 mpbs. Upload speeds nearly doubled to 3.8 mbps.
Also: I upgraded my wireless modem to the Cisco E4200 v.2 (about $150 at MicroCenter). It cooks!
My connections and download speeds are flying now.
Since you seem to be a Mac person, you might also consider the Apple Airport Extreme or Time Capsule models. I use a 1TB Time Capsule here, works like a champ. It also works just fine with Windows systems (my network is mixed Mac, Windows, PS3, Linux, iPhone, iPad, Apple TV, HP network printer, Toshiba TV, etc).
The Netgear WNDR3700 is also getting a little long in the tooth, the WNDR4000 and 4500 are its successors.