How can I tell what version of Linux I’m using?

Often times I will ssh into a new client’s box to make changes to their website configuration without knowing much about the server configuration. I have seen a few ways to get information about the system you’re using, but are there some standard commands to tell me what version of Unix/Linux I’m on and basic system information (like if it is a 64-bit system or not), and that sort of thing?

Can I identify my RAM without shutting down linux?

I’d like to price some new RAM for our in-house VMware testing server. (It’s a consumer box we use for testing our software on and running business VMs). I’ve forgotten what kind of RAM it has and I’d rather not reboot the machine and fire up memtest86+ just to get the specs of the RAM. Is there any way I can know what kind of RAM to buy without shutting down linux and kicking everyone off? E.G. is the information somewhere in /proc?