When I use more than 4gb ram on win7 pro 64 bit my computer restarts at starting windows...

When I use more than 4gb ram on my computer it restarts at starting windows screen. I had windows 7 pro 32 bit before, today i upgraded it to 64 bit. I did this so I could use the 6gb ram I had. When i was using 32 bit, my computer would load up with 6gb ram, it would just say in computer properties that i had 6gb ram installed(3.25gb usable). In 64 bit, when I put 4gb ram in the computer, it loads up, but when i put 6gb ram, it restarts at starting windows screen. When using 32 bit, my computer will load up with 6gb ram, in 64 bit it doesn't. Please help me.

My computer specs right now:

Processor- Intel® Pentium® 4 CPU 3.00GHz

Installed memory(RAM)- 4.00gb (because i can't use more than this)

System Type- 64 bit OS

BIOS- Phoenix ROM BIOS PLUS Version 1.10 A07 (latest)

Graphics Card- ATI Radeon HD 2400 Pro

And also, when i try to install latest drivers for my graphics card my computer goes to a blue screen and says windows has been shut down for security reasons, then it restarts.

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