You can refer to Windows 7 system requirements : http://windows.microsoft.com/systemrequirements. Windows 7 system requirements are lot less than the one for Vista. If your system are made for Vista, than running Windows 7 will be no problem at all. - Dynashox -
If you have 1GB ram then its obvious that your other system requirements would be ok. You can install 32-bit Windows 7 on it. - Mr.President -
Windows 7 should work on your Vista machine just as well if not better. (To me Windows 7 looks like they took something that is "ain't broke" like XP, and revamped it knowing XP rocked.)
the most important thing is the Ram if it is 1GB then you can install. for more you can visit http://techinfo.co.in/ this has helped me a lot in my installation. i hope i will do it for you as well.
It is not any problem if your system is of less configuration but you should need good RAM like atleast ! GB if it is less than this your sstem will hang always.