Usually proprietary desktops like the Sony PVC series can not be upgraded or overclocked to the extent you and i are hoping to do so. Just in case you are still or anyone is considering this I share the following:
I have a Sony VAIO PVC-RX752 that has an ASUS P4S266-VX motherboard Intel Pentium 4 2.0 CPU 478 socket; 512 MB RAM (upgrade to 1.5 GB max).
By mistake, I now know that these computers can be unlocked from Sony’s proprietary grip. While replacing the CMOS battery and upgrading the RAM to 1.5 GB I had the computer disassembled and after replacing the battery I put it back together and turned it on and instead of the norm, I heard three loud beeps and immediately realized that I forgot to install the RAM, so I just wanted to turn it off at that point but before I do, I see the BIOS screen appear but it looks a little different. And so it is that you can set the multiplier and bus speed etc. and access a more in depth BIOS by powering the computer sans the RAM modules.
Long story short, I too wanted windows 7 on an old Sony desktop, but couldn’t do so because of power requirements and I wanted to upgrade the CPU and RAM as well. By mistake, I found out that there is an alternate BIOS that can eventually give you the end result you are looking for. I am in the process of doing this now so I can not tell you the end result but I can tell you that I currently am waiting for a bigger power supply (above comment is correct in regards to Sony using not common size everything) in order to power a 512 MB Nvidia AGP video card and a CPU rated at 2.8 GHz (I should receive both in a few days)
So I will try and remember to update this post when all is said and done. Currently I have the same computer (PVC-RX752) running at 2.4 GHz (overclocked) with 3 GB RAM (1.5 GB was supposedly the max) computer is stable and if I manage to upgrade the pwer supply in order to update the GPU, I will be pretty happy with it. Maybe it will accept registered memory and i could have 6 GB RAM (three slots) haha wishful thinking i suppose but so was installing windows 7 and overclocking this guy in the first place ;)