To be perfectly honest your specification is more than enough to get you started. From the ahrdware point of view the only immediate weakness is your operating system. I am afraid you will have to give in to the XP Upgrade. When you do, remember to format your hard rives to NTFS, as this will overcome the dreaded 4Gb limit. As far as the capture card goes, then you will need an OHCI complaint Firewire card.
In the loger term though, assuming you are moving toward a professional operation, then look to take advantage of the multi threading options of Pentium 4 and other chip sets. Although this would almost certainly mean a new PC.
Softare wise, you will find an awful lot of support available for Adobe Premiere. If Premiere is too costly, then Ulead Mediastudio Pro is also worth a look. The important thing is to take tiem to research the products and fidn out what suits your needs best. Marc Peter's post on editing software links is a good starting point for your research. This post is at
As for the camera, then hows much is your budget. At the 'prosumer' end of the market then the Sony TRV950 and Canon XM2 are both worth a look.
Hopefully others will be willing to advise of their own experience.
Good luck with your venture!