You can simply buy all the parts. and everything and bring it to a local Computer store.. they will put it together for you.. for a small fee... and yes it should be upgradable.. but HP and Dell like to make their stuff hard to get to.
i agree, do some research and buy your parts then atleast you know wether you can upgrade later on, take it to a local computer shop and they will put it together for you
OR
(if you cant be bothered researching or dont know about parts)
go to a computer store and request for them to build you a computer that suits your needs and that can be upgraded later on and get a qoute