Buy a new pc or upgrade my current one?

Ok well basically i have a dell inspiron 4800 it has an Intel Pentium 4 CPU clocked at 3.GHz with 1 GB of ram and brand new graphics card from nivda. I was also going to upgrade it to windows 7 and 3GB of ram. So as you can see it would be high end but is an old pc so the processor and what not are worn out or getting there or do you think i should upgrade to http://configure.euro.dell.com/dellstore/config.aspx?b=&c=uk&cs=ukdhs1&kc=HDP1&l=en&m_30=111787&oc=D004516&rbc=D004516&s=dhs or do you think it is a downgrade? What are your views? Upgrade or update