Win 7 32-bit will address up to 3.4 GBs of RAM (install 4 GBs).

Win 7 64-bit can address much more RAM. 12 GBs, 16 GBs, 24 GBs - depending on your motherboard. Check the specs on your motherboard from the motherboard manufacturer's site.

For the Pentium D, stay with the 1 GB graphics card that you now have.

This will explain well: http://msdn.microsoft.com/en-us/library/aa366778(v=vs.85).aspx - but remember, your motherboard determines how much and what type RAM can be installed.

More info here: http://social.technet.microsoft.com/Forums/en-US/w7itprohardware/thread/048ed81d-0e9a-471c-99fa-ca6c26c956d1/
 

You need to specify in your first post that you are talking about video memory, not system memory. The hint that hinted this to me was that you said "radeon".

Now what you need to know is that ram size in a video card is generally irrelevant and has nothing to do with the performance of the card.

Now, would you care to give us more details on your situation, your system setup, and what you need?