i agree with lucassix. your computer is more thn decent and shouldnt have any problems with running anything more. from your post, i gather hints that you may want to use programs such as windowsblinds and maybe even litestep. these can both be run on the computer you have talked about. maybe some maintanence would clear up some speed issues. a defragmentation of your hard drive, adware/malware/spyware, virus scan(s) would be something that i recommend as those can speed up a computer exponentially.
a ram upgrade is also something i recommend as many gui replacement programs are very memory dependant unless you get a shell replacement which in my experience have run well on low-spec computers.
if you are intent on acquiring a new computer, the website lucassix recommended is great. they seem to have great computers for way below retail price. my only word of warning to you is to STAY AWAY FROM DELL. they are the scum of the computer world [i kid] and should be avoided at all costs[ i dont kid, and you will most likely save money by avoiding them]
once again, i think you should get more ram [ relatively cheap wherever you look] and maybe a decent video card as your current is integrated and is sometimes not suffiecient to run gui replacements, and other things as well. your computer most likely came with an open AGP8x slot and you could fill that for about 100$ [US/CAD] with a decent video card. otherwise, the only other thing i could think of is maybe a hard drive upgrade f your current is getting too full or if those other files [ from the external hard drive] would not fit on your 180 gig, is to get anohter hard drive. your computer is most likely sata compatible and i recommend those drives becuase most of them are very cheap [good quality though] and are usually priced around the 1.30/gigabyte range so are easy to integrate into a budget.
read it!
woot, mastermind