vdimitr,
First, let me address Cman's concerns. It appears that your CPU and mobo are 64 bit. So XP64 will be OK on your system. The current problem is that there is, still, very little software that has been ported to 64 bit. If you've already purshased XP64 then, I guess, its OK to go ahead and install it. Just remember that you still won't notice any big advantage until the software you use is also 64 bit.
OK, on to your questions:
Plz advise if it is possible to install winxp64bit over the already installed winxp32.
Yes, it is possible to install 64 over 32. But, IT IS NOT A GOOD IDEA! Personally, I wouldn't do it. You're just asking for problems. If I were you, I'd back up all necessary data and then repartition (if necessary) and reformat. A bit of a pain but its 8 to 10 hours for a reformat and reinstall vs. who knows how many hours of frustration trying to fix niggling problems with the install. Just go ahead and reformat. Its easier in the long run.
And if this should solve 2 big problems with 32-bit: first the 500GB WD Sata HDD is found as 130GB...
I've only seen this once before (on 32 bit XP) and it had something to do with installing the correct SATA drivers. You should have a CD that came with the drive. Read the instructions as you may have to pull the power cable on the drive and install the drivers before you install the drive. Read the instructions, they'll tell you.
We just discussed that in the Tech section a little while ago. Let me find it and I'll put a link back in here.
OK, here's the link to the RAM question:
http://www.nudecelebforum.com/t22703-i-got-4gb-ram-but-the-os-only-see-3.html
Good luck and let us know.