Linux – What resources are available for building a cutting-edge Linux box

First off, you should take a look at Ubuntu with the WUBI installer. Now lets answer your sub-questions :).

Can you partition a hard drive which already has an operating system on it?

Yes, and most Linux distributions include tools that will do this for you easily, and without negatively impacting your Windows data right inside the installer. WUBI will install Ubuntu just like any other Windows program without impacting your partitions at all.

What is the most user-friendly Linux Distro?

Ubuntu is probably the most popular at this point, I recommend it strongly as it is built on a very solid base (Debian) and it was designed to be user-friendly.

How can I back up my entire hard drive?

I would get an external USB hard drive and copy data to that. Thank you for considering this before you start!

Will my hardware work with Linux?

Probably. Most common hardware is well supported, and improvements are made in this area every day. Ubuntu's wiki has a comprehensive guide on hardware compatibility.

Hardware compatibility is a hard problem on all Operating Systems, not just Linux. Linux just has a reputation for not having strong hardware support. This is a fault of the hardware manufacturers, not Linux.

Supported capability on your clients

iw phy on GNU/Linux should list what you want (and a lot more) about your wireless interfaces, although it's a bit confusing at first sight.

What you're looking for is if your network card works in dual-band mode. You'll then see Band 1: and Band 2: section, first one usually for 2.4Ghz and second one for 5Ghz. Under each bands you'll see Bitrates (non-HT):, listing the supported bitrates for 802.11bg in 2.4Ghz or 802.11a in 5Ghz.

You're also looking for HT20/HT40 under Capabilities: for each band. It means 802.11n is supported. HT20 is for 20Mhz width channels, HT40 for 40Mhz width.

If you want to know more about 802.11n capabilities, for example number of spatial streams (for MIMO) and supported rates, look at the HT TX/RX MCS rate indexes supported: line. 0-15 means MCS Indexes from 0 to 15 are supported, ie. it can works in MIMO 2x2 with data rate up to 130 Mb/s in HT20 or 270 Mb/s in HT40. Additionally, if RX HT20 SGI and/or RX HT40 SGI is listed under Capabilities:, it means Short GI (400 ns) is supported so max data rate is 144 Mb/s for HT20 and 300 Mb/s for HT40. See here for a list of MCS Indexes: https://wireless.wiki.kernel.org/en/developers/documentation/ieee80211/802.11n

Frequencies: list supported frequencies and associated channels. Some may be disabled because either your hardware or software doesn't support it, or because your regulatory domain doesn't allow it. If your WiFi access point broadcast signal on a disabled channel you will not be able to connect. You can see which channels are allowed in your area here: https://en.wikipedia.org/wiki/List_of_WLAN_channels

Additionally you may see VHT Capabilities and other VHT related info. It's about 802.11ac, but your access point does not support it (neither the chipset you indicated), so you can ignore it.

Access point configuration

I would really recommend you to keep your WiFi access point configured to broadcast on both 2.4Ghz and 5Ghz. Nowadays, 2.4Ghz is very crowded and is more subject to interferences, when 5Ghz has much less devices using it. Problem is that 5 Ghz is generally supported only on recent or high-end devices whereas 2.4 Ghz is the default band used on WiFi devices. Also 5Ghz signal range is less than 2.4Ghz and more subject to degradation due to obstacles.

Using both bands enables you to have wireless on the most devices (either because they don't support 5Ghz or because the signal is degraded, so they fall back on 2.4Ghz) while having better performances in high-end/recent devices (because they're using the 5Ghz band).

Finally you shouldn't worry about B, G and N on your access point, you should select the mode which provides the three standads (mixed) so that older 802.11g (802.11b devices are very rare nowadays) devices can still connect and newer 802.11n work at full speed. Also only 802.11a and 802.11n work in the 5Ghz band. (there's 802.11ac too but you're not working with it)

Maybe you'll also see a channel setting. Usually leaving it to the default will do the job, your access point should be clever enough to select the best. (and it prevents you from selecting a channel overlapping on others creating even more interferencies)

Link between the two

iw dev on GNU/Linux will show you how your client and your access point are connected. Ideally it will be in a channel on the 5Ghz band (and allowed in your area) with 40Mhz width.