My system was also slows down when I boot Windows 7 64bit with 4GB RAM. I was using Windows XP 32bit with 4GB RAM from past 1 year with no issues. I played heavy games, installed heavy software on my system, and my system had no problems with 4GB RAM.
My system's specification are given below:
Processor: Intel Core2 Due 2.4GHz (6600 Model)
Motherboard: nVidia P6N Platinum Edition (nVidia 650i SLI Chipset)
RAM: Kingston 4 x 1GB, 800Mhz (All RAM have same speed and brand)
VGA: eVGA (nVidia) Geforce 8800 GTS 320MB version
I also tested Windows 7 RC (32-bit), and also found no issues with 4GB RAM. But Windows 7 (64-bit) had this issue with the above hardware.
I was able to solve my issue. I just updated the BIOS on my MSI nVidia P6N Platinum Edition (nVidia 650i SLI Chipset).
Now Issue is resolved. Now Windows 7 start with normal speed with 4GB RAM. So other users should update their BIOS, and try again!