3 Macs in Household Having Slow Wifi Internet Speeds, While PC is Fast?

I'm having trouble trouble-shooting my brand new 2011 Mac Mini (base model: 2.3Ghz i5, Intel 3000 Graphics, 4GB RAM, 500GB 5,400rpm HDD). Also, I've noticed this issue is prevalent on all three macs in my household (2008 macbook pro, 2009 macbook pro, 2011 Mac Mini).

I have Comcast Cable Internet, connected to a Linksys router which reaches my upstairs 7-year-old Dell (has a PCI Linksys wifi receiver installed) just fine. Fine enough to get pretty fast speeds, I might add. If you find my Dell's specs relevant, they are here: (3.4Ghz Pentium 4, ATI Radeon x850 256mb, 140GB 7,200rpm HDD, 5GB RAM)

I first noticed the issue when attempting to download a file in Safari. It's a 5.8MB file and I noticed it was going to take several minutes to download... It was downloading at ~40kb/s (See Screenshot). I found this funny, so I tried downloading the same file on the Dell, which sits at the same desk, literally right next to the Mac Mini. The Dell downloads the file at ~650kb/s, taking but a second to download the file. This is an astounding difference. I cannot for the life of me figure out why this is happening.

The issue is not Safari, as I noticed this issue while downloading updates from Apple's Servers as well. Downloading a 70MB file, approx, took about an hour. Also, downloading apps from the Mac App Store took hours upon hours, as they were several hundred megabytes... Trying to get websites like Hulu to stream video well was a lost cause.

These computers are connected to the same Wifi Network, thus getting the same cable internet, so the issue is not the internet service (The Dell doesn't have speed issues), or the wifi signal being weaker (the wifi signal on the Mac Mini is full bars).

I've been trouble-shooting this thing for days. I started noticing the same issue on my 2008 Macbook Pro after attempting the download with it as well, and with another 2009 macbook pro in the house (same problem). The "solution" I found came from something I read online (unfortunately google does not show many results for issues like mine, as it is apparently unique). One of the few relevant results I got was from 2006 about a Mac connected to wifi that was having slow internet while a PC also connected to wifi in the same room was not slow. It sounded very similar to my issue. After reading responses, someone suggested to turn off the bluetooth. Although it sounded absurd, I tried this on the macbook pro. No difference. Tried it on the the Mac Mini. This did not change the download speed.

I posted on a forum with my problem and someone suggested updating the firmware on my router, and if that doesn't fix the issue, to try changing the channels my wifi is transmitting on. I did both of these suggestions (including changing the channels to the recommended channels 11, 6, and 1). This did not change the Internet/download speed on any of my macs.

So, considering this is the Apple Support Forum, I figured somebody here HAS to know enough about these computers to be able to help me out. So, for anyone who's feeling generous, I'd really appreciate some help and advice.