(edit) Feeling like a dog for not mentioning summin that's already under ya nose. The Magic Mint editions ... honestly you might already have everything you need installed right now. Just by tweaking the Mint you have.
I had Mint 10 main ( with gnome, xfce and fluxbox installed.) When I was using fluxbox it would idle at like 80mbs/ram. That's Mint 10 32bit, their lighter weight stuff no doubt runs much lighter and better than that outta the box.
So ye may be doing yourself and Mint a disservice by distro hopping. Might want to look at how to tweak whatcha got. Cause although I haven't used it. Don't doubt Mint's lighter stuff can be made to match pretty much anything else going.
ie: Use a lighter DE, shut off unnecessary startup apps, download bum and disable any unnecessary services that start when ya boot up. Tweak your visual effects to settings that use less. Change ya swappiness to vm.swappiness=10 to cut down on disk writes.
Recently one of the Linux folks on the forums who no doubt knows a thing or 4 about Linux ( exploder ) introduced me to a snappy lil distro called Slitaz. Am actually using it more than the others lately. Fascinating and peppy Linux distro. Total install on my sys is under 500mbs so far. Tiny distro ... good on sys resources. But don't let the size fool ya. It packs a punch !
( slitaz has a variation of firefox called Shiretoko ... Based on FF 3.5.7 or so.) Also comes with a default browser that's awesome on sys resources called midori. But midori is buggy, crashes sometimes ... Locks up and pushes cpu to 100% sometimes. Might be sorted in the newer versions of it though. I had to nix it ... didn't feel like messing with it anymore.
AntiX11 is top notch ... Has a variation of firefox called iceape. Really resource friendly distro as far as I've been able to tell pretty much all the way around. Lightest, fullest featured distro I've yet used. Which being a newb isn't saying much.
Crunchbang get's a lot of positive feedback and promotion from what I've seen and came across related to it. Unlike ubuntu ... guessing CB, actually lives up to the hype.
Haven't used CB yet ... based on some resource stats I came across don't really see much point. AntiX is no joke debian based. CB no doubt also very good ... also debian based. Seriously doubt CB default matches up to antix though.
Puppy's already mentioned. One I'm thinking about installing alongside all the rest sometime soon. Absolute Linux 13.37, it's based on slackware and is supposed to be a great fit for older/lower resource hardware ... Which like you, I'm in the 512mb/ram club. Sighs ...
Absolute isn't supposed to be exactly flashy ( eyecandiwise) or uber pretty like the heavy weight bloated stuff. But didn't at all look to be ugly either and if performance is the main concern. Think it's worth looking into.
Don't doubt Mint's light editions could be turned into grease lightning and feather weight sys resource users if someone does a lil bit of homework and tweaking. The Magic Minty developers are no slouches. Just a thought etc ...