Whats the difference between Ubuntu & Xubuntu 6.10?

Ubunut uses Gnome GUI while Kubuntu uses KDE.
Windows users will feel more at home using KDE.

Yes. Xubuntu is for low end machines.

It is recommende tht u use Ubuntu 1st as is the most stable.

Xubuntu uses the XFCE Desktop Envionment, simple easy and light DE that can be customized a lot.

XUbuntu Fiesty doesn't seem a lot different from GNOME(differs only in RAM usage )

What's the System Requirment to Install Ubuntu 7.04?

You should need 512MB of RAM and I guess something like 800Mhz to 1Ghz processor is the lowest you can go on Ubuntu so it's smooth enough. If you have that I don't think there would be any trouble. If you got less RAM then that go for Xubuntu. If your processor is slower than that, there are other distributions you should be considering IMO.

And yeah the requirements slightly increase with beryl/compiz, you'd need a better processor. I think 512MB RAM should be sufficient though I have seen beryl running on 256MB RAM out of which 8MB was reserved for onboard graphics. It was an Intel 915 chipset board.

thanks tech.... My System Has:
Intel P4 3.06 Ghz 533 MHz with HT Enabled
256 MB DDR RAM 400 MHz
nVidia GeForce 2 Based Graphic Card with 64 MB Memory (AGP)

Can i run Ubuntu 7.04 or Fedora Core 6 in this Config???

Go for Xubuntu or if you're installing Fedora then install it with XFCE desktop. Don't install GNOME or KDE. I think you should consider adding RAM, if possible. The RAM prices are quite low these days. 1GB DDR1@400Mhz should come for around 3-3.3K so if you can add that much RAM, not only will gnome/kde be smooth but beryl will run nicely too.

currently , DDR2 is about 0.6-1k cheaper than DDR RAM

I bought a 1gig ram for around 3K.. Now my system is really fast..

You can also try Vector Linux Standard Edition. It comes with XFCE preinstalled.. Its a nice distro if you like Slackware