I would try it with what you have. OPs have said that single SSID works pretty well with today's gear.
Been my experience. Works wonderfully and across several manufacturers gear (all 11n stuff). Caveat, InSSIDer can cause some oddities when actively running it with things like roaming (see recent experience I just posted about).
Since you already have it, give it a try. Yes, you generally do not want to overlap Wifi channels, at least if you want to maximize wifi throughput if you have multiple devices on seperate APs/routers all running at the same time.
My setup is Netgear 3500L in basement on one side of the house, channel 11 2.4GHz. TP-Link WDR3600 on the other side of the house on the main level, channel 1+5 40MHz 2.4GHz and Channel 146+151 40MHz 5GHz. TP-Link 841nd in garage with antennas outside Channel 11 20MHz 2.4GHz. There is effectively no physical locations where the basement and garage/outdoor APs overlap their wireless signal, so zero interference and since the main level AP covers the area of the house that is primarily used, or at least mostly used when you want really fast coverage I figured on setting it to 40MHz 2.4GHz (I have no neighbors close enough for their networks to interfere with mine or vice versa. Even outside my closest neighbor is >>40dBm below my outdoor Wifi network's signal strength).
I will shortly have another WDR3600 to replace my ageing Netgear 3500L, as I got a used 3600 a few weeks back to replace another 3500L and it has been working wonderfully, so I figured on replacing my main router with one too. $36 new in box seemed worth it. That'll let me boost the basement/that side of the house main level to 40MHz on 5GHz, though leave 2.4GHz as 20MHz.
Roaming has been working great. Nice and seamless from my perspective (and more importantly my wifes). All my devices tend to roam quickly, from about 2-3s for my laptop to around 8-12s for our iPhones and my wife's iPad 2 (but never in danger of losing a connection or having a really poor connection). Things like video streaming, file downloads, etc all run fine with the later maybe seeing a brief .5-2s pause and then resuming as it roams. Things like facetime and skype I never see an interuption in.
YMMV and there ARE a few people who have reported problems. I find that so long as all access points are set to the same SSID and all bands are set to the same SSID, it works seamlessly. Start mixing (different SSIDs) it up though and some devices get real sticky either with their band or with the AP that they are connected to even though they are well below optimal signal strength and you are nearly standing on another AP.
