Honestly, it's not worth spending/investing additional funds on a Pentium 4 (P4) machine. I upgraded my P4 2 years ago to a budget system initially, and the performance gain was significant to say the least.
I would not upgrade your existing system. Rather build your own and you'll notice a major performance difference. If you're willing to build your own, please state how much you're willing to spend and we'll recommend you the components you should purchase to upgrade to a new system. Also please state what country your from, so we can determine the currency we need to adhere to.
To outline the above, I wouldn't spend any more money on your existing system. I'd upgrade to a new system all together, which can be done at an affordable level yet provide at least 5x performance over your current setup whilst more energy efficient too.
All the best.