I have seen many people overestimate the cost of upgrading Dell PCs particularly the 8400.
There is a 'buy it now' Pentium 4 670 on ebay now for 15 bucks. You can probably pick one up cheaper, by bidding in auctions. That will offer an appreciable difference to what AboveDaClouds currently has in his PC. I personally would not pay the extra money for a 672 unless you are confident you need and want hardware virtualization - other than that, the 670 offers identical performance.
Another thing people get wrong is the performance of these machines, thinking they offer worse performance than they actually do - probably because they have never tried to use one to run modern applications / games. I have put 300 hours into Elder Scrolls : Skryim on this machine, it runs fairly smoothly on high settings. Granted I have a 1Gb graphics card in my 8400 now and a 500W PSU, but the cost of ownership over almost 9 years has been low, in my opinion.