The problem is Windows 7 will only let you "upgrade" to the same version or bit level of Vista you have now. If you want to install 64 bit you will have to do a complete new install, either to a reformatted hard drive, a new hard drive, or (the least desireable) a partition on the existing hard drive.
Even with Vista 64 bit installed, I went from Vista Home Premium to Windows 7 Professional and had to do a complete new insttall too. It wasn't really a problem as I had bought a new hard drive to do that anyway.
Dimension E510 with D925 CPU and Vista Home Premium 32 bit
Home Built PC with Q6600 CPU, 8GB RAM, HD4850 Video, HTO Striker 7.1 Audio Card - Windows 7 64 bit Professional
Sonar 8.5.1 ( Producer) DAW Software/Focusrite Saffire Pro 40 Recording Interface and Frontier Tranzport wireless controller.