You can play with the compatibility settings or use XP mode.  Most problems with the older games have to do with graphic driver interaction.  Vista and Windows 7 share the same kernel code base.  I believe RF 4.5 and RF 5 are similar code base, but you would have to talk with someone at knife's edge.  I run windows 7 64 bit with an ati 5770 and a phenom x3 and have no problems with RF5.  I have everything cranked up and she runs smooth. 

If I remember right in-place upgrade of XP to windows 7 is a no go (as they are dissimilar kernel base).  You might have to do a fresh reinstall, thus reinstalling everything else.  Check the Microsoft forums.  Hope it all works out for you!