I've been reading this forum for a while and I think it'd be the best place to ask for an advise.
I've been running Linux on my main PCs since 2001 when I was (still) using Slackware from the '90s days.
Every PC I've got never run it's original shipping OS. I used to turn it on, check the BIOS booting options, slip a Linux CD inside and then install it. No four-color-flag, then.
I am currently running Kubuntu 10.10, but have tried also Ubuntu, Gentoo and Fedora. None satisfied me. This is why.
Mine is an ASUS G1s laptop, T9500 (x86_64), 4GB RAM, 320GB SATA@7200rpm I use 50% of the time as personal computer (browser, CD/DVD burning, OpenOffice, ...), 50% as a development platform (C/C++, Postgresql, ...). I would say it's quite powerful for me.

First point. Responsiveness.
It's a nightmare. Copying large files back and forth to a couple of USB disk will make the system unusable, even to edit a .cpp file with vi.
The same happens when burning DVDs or unpacking large tar.gz or rar archives. I can run ionice(1), but only at the CLI, no (easy) way with the GUI.

Second point. GUI.
At the moment I use KDE, but there's no problem to switch to another DE.
Both KDE and GNOME as shipped by Ubuntu are either buggy or incomplete. At least to me. I'm not sure whether all these issues are in the mainstream or have been introduced by the Ubuntu packagers. I used to be happy with KDE v3, but that's not available any more.

So what I'm looking for is an OS with a decent responsiveness even during large I/O loads (I don't mind if the copy will take 5 or 6 minutes, but I do mind if I cannot type code into vi).
Then a decently complete DE would be greatly appreciated.

Any advise for me?
Thanks a lot in advance.