Opening and parsing large files has been and probably always will be, a relatively time consuming process.
An excel 2007 file will be (I assume) in the new XML based format, and at 50mb will contain a lot of data. As has been said, the 50mb will be loaded into memory almost instantly, probably aorund a second with a good hard drive. However Excel then has to parse all the data, this is what takes the time.
Unfortunately Excel doesn't yet make good use of multiple cores nor is it particularly fast to parse its files.
The problem lies with Excel and not your PCs in this case I fear.
EDIT: Also, a 64 bit OS will, in most scenarios, reap no performance difference. If you think logically about how a 64 bit OS differs from a 32 bit OS, it is not in HOW it processes data, just that it can address more memory locations at once. The primary advantage of a 64 bit OS is simply that of being able to recognise and physically utilise more memory. For memory hungry applications this is very useful, however for many commercial apps, they are all still 32 bit, so even when run in a 64 bit environment, they will have a 2-3GB memory limit anyway (depending on how it was compiled). Having said that, if you have 4GB of RAM in a machine, it make no sense to have a 32 bit OS when it can be avoided, purely because from the end users point of view it will make no difference and it will give them access to a further .75GB of RAM.
