A couple things to consider:
-Almost every CAD application is single threaded. More cores only offer a limited amount of boost, but more clock speed will help. It should not be a night and day difference though.
-RAM is easily upgradable as long as you are on a 64 bit OS. Upgrading to 16 gigs of ram will cost under $100. If you need to upgrade to Windows 7/8 then it’s another $100-2oo for the software upgrade
Since you’re spending someone else’s money, put together the justifications as mentioned. It also helps to point out if your computer is nearing the end of it’s service contract.
…and then pour a bottle of coke in it if all else fails.