A quick search on this site will show I have been advocating Mint Linux for quite some time, and I am hardly the only one. On a rather old PC running and early Athalon processor (single core, don't recall the speed though), I use the XFCE interface version. It has been flawless. Apparently there is also an even lighter interface by using the Fluxbox version, but I haven't tried that. I am quite happy with the XFCE version and it's development.
I have always heard a lot of good things about Mepis, but it has never worked well for me.
As to the argument about Linux not appearing to be better than Windows 7, that is because it isn't currently. Windows 7 is very fast and streamlined for an OS. The only way you will find a Linux version that is faster than Windows 7 at this point is to get one without a GUI, or at least without a GUI with all the same bells and whistles (you can accomplish the same thing with Windows 7 by the way - turning off the GUI and getting better performance, but who would want to?). As Zane has pointed out - at this point you don't go to Linux for a windows-like experience with better performance. You move to Linux for the flexibility, price, and/or desire to learn a new system.
Having worked extensively with every major OS type out there (except Mac, I have worked with it but not extensively), I can say that I generally like Windows best, but Mint is my preference due to cost and flexibility. Windows has the best software base and is, therefore the most usable in my mind. Mint, however, is nearly as usable, free (though donations are greatly appreciated by the developers), and has great support as well. zO/S is a pain on a good day, but supposedly is one of the best for non-Parallel High Performance Computing (HPC). The only good thing I can definitely say about it is it really helped me get a new, fuller appreciation for virtualization (something I have been strongly interested in ever since I first heard about it). Sun Solaris is a nifty OS for Unix, but always crashes a lot for me. I think it is because I don't use it to do the same thing every day, but rather fiddle with it all the time. It is a "Cranky old man" OS, but I like it none-the-less. Last but not least, Macs are simple to use and slick, but are very expensive and not nearly as useful as a general use machine from what I have seen. Again, however, I have not used them extensively so don't blast me Apple-fanboys
, I just want to give my first impressions as it were. Hope that helps any readers. If not, you are done now.