I have a inispiron 1545 dell laptop,3gb memory, running the 64 bit version of vista. Was looking to upgrade to windows 7. Should I upgrade to the 64bit or 32bit version of windows7?
Also was thinking about upgrading mr ram to 4gb, do I need to replace all my ram with new matching pair of 2gb ram chips or can I just change the 1gb for a 2gb. Was on crucial.com and they said that the matching pair will give better performance. Can someone give me some advice please?

As for the ram according to your system specs if I'm reading this right you have 2 slots so to upgrade to 4 gb you will have to replace the 1 gb stick with a 2 gb stick. 3gb should be plenty for 7. As for which is better (64 bit or 32 bit) I don't know about that. However to upgrade I believe that you have have install the 64 bit version of whatever version of Vista that you have now (ie home premium to home premium, buisness to pro, & ultimate to ultimate). To go to a 32-bit or to a different version you will have to do a clean install.

Go with 64 bit.

If you up the RAM get a new matching pair of 2 x 2GB or else just run it as it is.

I am running Windows 7 Ulyimate 64 bit and it will also use 32bit software e.g I am also running MS Office which is only 32 bit.