If you are a serious trader, here's the spec and help you want.
1: Hard drive size and speed -not really important. 20 Gb will last you forever. Best value for money should be 40G/7200.
2: CPU don't buy the very fastest- it will rob you of money for nothing extra. AMD ( my favourite) 1700-2000 is plenty fast enough. ( My system is 1700). Or intel P4 17/1800.
3:512Mb memory PC133 spec is plenty good enough. Leave the DDRAM for the games players at 50% premium.
4: Only have Windows XP Pro. forget the rest including XP Home.
5. Motherboard is unimportant- they are mostly 6 of one and half a doz. of the other. Again , no need for anything fancy here like RAID. On board sound is standard. DON't get one with everything on board.
6. Video card. Chose a DUAL head AGP such as Jetway 315 128M. I know these work in multi monitor setups. There are plenty that don't.....This will enable you to expand to several monitors....
7. Don't use USB for ADSL or Broadband. Fit a network card.
8. With windows XP , you can fit another network card and hook up another PC to share your internet connection. However a better way is to use a broadband router. Doing it this way, if one PC suddenly dies, the other PC will still be live on the internet. If you share through a network card, and the main PC dies, the other PC can't get on the internet.Another bons with a router is the inherent built in firewall- essential on broadband.
9. Fit a 56K modem as backup just incase the broadband goes down.
When you have digested all of that, go and find a local independant PC shop, show them this spec and ask them for a quote. You should get change out of £500. Why? because when it goes wrong ( and PC's do go wrong) you won't be lumbered with things like £1 a minute help lines run by dorks who read from a script . You won't get " send it back and we will wipe your hard drive free of charge "........ you had valuable data on there? oh sorry..... You wont get fobbed off with " sorry madam, it must be your software" or "sorry, this PC is not designed to go on the internet all day". It's TRUE, I've heard it all before.
Why not build it yourself even ?