For the record, my laptop with 2 GB of RAM and a 1.8 ghz Intel Core 2 Duo runs Linux Mint Nadia KDE just fine. No lags. Boots in less than a minute. I run Firefox for my internet browser. Now, I used to fret about how ancient the thing was, so I used to run Mate on it for that reason, but I got a bit annoyed with that. Many people like it however, but I ran into various problems that probably only affect a small number of users... But sure, there's Xfce and Mate if you are a heavy multi-tasker. I don't really multi-task that much, other than running VLC in the background to play music while I browse the Internet or play Dungeon Crawl. But if you are decoding nucleotides, brute-forcing encryped strings, or analyzing satellite data with your spare cycles, then you might want a lighter weight desktop. I just think 2 GB is plenty for any flavor of Linux I can imagine. I have never heard of a Linux distro that needed more than 2 GB. But I guess it depends on what you have in mind. I know that in KDE, there is something called "Workspaces," that allows one to switch between working configurations, i.e. open apps. I don't use that myself, but if you did, then that particular feature might well require a bit more than 2 GB RAM.
My desktop runs 64-bit Kubuntu 13.04, my htpc runs 64-bit Linux Mint Nadia Xfce, my answering machine runs 64-bit windows 7, and my laptop runs 64-bit Linux Mint Nadia KDE. Each seems suited to its purpose.