I'm not precisely an expert, but I have built my own system from scratch, so I know a bit about components. Bottom line, to go from a 1 Ghz P3 to a 2 Ghz P4, you are talking about an almost entirely new system. You will have to replace, at a minimum, the motherboard, processor, RAM, and power supply. Since you want a new graphics card as well, you're talking $500 to $600 dollars as a rough estimate, if you want quality components. You can skimp on the power supply but will pay for it later when your system dies. Also, your Packard Bell case might have a proprietary design that won't allow you to mount a new motherboard, which means a new case as well. Add a new hard drive and CD-ROM, and you have a whole new computer. You've waited too long to upgrade, right now any upgrade that will bring you anywhere near current speeds will cost more than it's worth. The good news is that you can put together a new system for less than $1000 dollars, way less if you use your old monitor and keyboard/speakers. I gained all of my computer knowledge from reading Maximum PC magazine,
www.maximumpc.com , I highly recommend it if you can find them in the UK. They did an article on how to build your own PC two years ago, I used that and a little help from friends to get mine up and running. A google search should yield lots of pages on putting together your own system. It's not difficult at all really, you just have to be careful and patient. A good source for components is
www.newegg.com and
www.monarchcomputer.com ; I've bought from Monarch before and been very happy with their service. Newegg has a very good reputation as well, I plan to buy a copy of WinXP from them soon. Good luck with your upgrade.
Edited to add: BTW I strongly recommend Athlon XP processors over Intel Pentium 4s. Unless you want the fastest on the market, Athlons are much cheaper and perform just as well. I'm running an Athlon XP 2200+, which is comparable to a 2.2 Ghz P4. You can find Athlon chips several grades higher than mine for well under $100, and a good motherboard for about $80. Comparable P4s will be closer to $190 for the chip and about $90 for the motherboard.