You sound as if you want a lighter-weight GUI more than anything else. Inevitably, that means giving up a few of the luxuries (some of which may be helpful luxuries and some of which are just flashy effects).
I think you should try to find a lighterweight GUI that doesn't feel like sackcloth and ashes. XFCE is still pretty decent, Windowmaker, Fluxbox are in the same area.
Enlightenment is an odd combo in that its still quite flashy, but lighter than the biggies (Gnome, KDE).
LXDE gets good comments from people who really like a stripped-down desktop.
Once you have done that, it will help you choose a distro, because there is no real point in choosing one that doesn't have the desktop that you want in its repos. You could try grabbing the XFCE packages for your existing Ubuntu install.
I could just carry on with Windows, but from turning on to Firefox/Chrome loading up takes on average 15 minutes, sometimes much longer.
Hmmm, 15 minutes sounds excessive, even for Windows (oh, Windows Vista, maybe??). How does Ubuntu (the Gnome Ubuntu, presumably) compare? These days, your amount of RAM is a bit on the low side, but should still be OK for simpler requirements, which is what you say that you have.
What browser are you using? Opera is my browser of choice, but firefox ought not to be too bad, if you don't try to open too many tabs (if you do go for lots of tabs, memory consumption can get excessive). Something like Dillo is very light, but does make a mess of the rendering on rather too many sites and its appearance is a bit clunky. Not sure about Chrome.