If you post the make and model of your cable modem (or even googling it with 802.11) should give you the info as to what protocol it supports. But unless yours is ancient it likely supports N anyway. Of course, it could be configured to disable N, so you might want to check that.
A quick way to verify if your router supports/is configured for N (or B or G or whatever) from a Mac is to connect to the network via wi-fi, then click the wi-fi/airport icon at the top right corner of the screen while holding down Option. It'll list PHYS Mode: 802.11_ for whatever mode it's connected with.
If yours doesn't support 802.11N and you rent your modem, or it comes free with your cable account, contact the company and ask if they have a newer model. Usually they replace free-of-charge with the new model when customers ask, as it's simpler for them when diagnosing support issues.
Also, don't dismiss what was mentioned earlier about antenna placement: It makes a huge difference. Relocating/changing the orientation of the modem or antenna can make a huge difference. There's some decent info at this link:
networking - Do different positions of the antenna on a wireless router make any difference? - Super User
If you can move your router to a different room, preferably one more centrally located in your house, that will make a huge difference.
As well, different wireless routers can make a huge difference in transmission distances. Generally speaking, built-in routers in modems are terrible, so adding a decent router could rectify the situation by itself. Also, some wireless repeaters/extenders/bridges don't play nice on networks with different brand routers, so that could explain some of the reviews you saw on Amazon.
Companies tend to rip people off on extenders/bridges, charging way more than they're worth and selling underpowered devices, so in general I wouldn't recommend buying a purpose-built wireless extender. You'll likely be disappointed. If I can't fix wireless issues by upgrade equipment or relocating it, I usually either buy a router that's 802.11N-capable and can run DD-WRT (which will allow it to run as a wireless bridge), or Apple's Airport Extreme/Express devices. If you're looking for simple setup, nothing beats Apple's Airport devices. The downside is they aren't cheap, but the previous-generation Airport Extreme is available for a discount on eBay et al.
If you really want to try a wireless extender, Netgear's WN2500RP is probably the best. It's about $80. Remember that you'll have to place it somewhere where it can get consistent, strong signal from your router.