not totally true, laptop you can definitely upgrade RAM, its pretty easy too. around as easy as upgrading desktop RAM i guess...
you can upgrade CPU and other stuff i think, but its a lot harder to find stuff that is compatible, will fit properly, and wont cause too much heat, so therefore isnt really recommended.
if its a celeron, its a single core. so yeah its understandable why its lagging.
put it in 640x480x16 and low details, then see if its still bad. you can upgrade video card, but if i was you i wouldn't bother with it, your CPU is too slow to gain any sort of real benefits. if it were the other way (like, you had a high powered core2 or amd X2) and a dodgy video card, then i would recommend it but since you got what you got, i wouldnt bother.