I know that if I want to network two computers, I need a broadband router(I'm hoping to get wireless) and a wireless PC card (do I)?

I have a main computer that I use with W2K NT Professional and one with Windows 98. I want to transfer several large files on that Windows 98 machine to the W2K one. Would two different operating system matters? Additionally, if one wireless router is needed, do I still need to buy two wireless PC Card for both of my computers?

If you only want to transfer files from W98 to W2K, you could work that with a W98 special serial cable (cheap but very slow) or a parallel cable (still cheap and a little bit faster).

The same -but faster again- can be achieved with the program LapLink (that comes with a transfer cable).

If you want to do a handyman-special, take the HD from the W98 PC and stick it temporarily in the W2K PC, to transfer your data (is by far the cheapest). Make sure to set the master/slave jumpers correctly and reset them before you put the W98-disk back in.

If you want networking, then if you go the wireless way, you will need a router with a built-in or a separate wireless access-point, and a wireless PCI-card for each PC. If you live in a densely populated area, go for the more expensive but less congested 802.11a (5GHz) standard, otherwise for 802.11b/g (2.4GHz).

It is cheaper and faster to go the WIRED way. You only need 2 10/100 Fast ethernet PCI-cards and a cross-over cable, which will cost no more than about $40.-/£20.-.

Tell us if you want anything more from that network (like internet/broadband)

So, basically, for the networking, I just need one broadband wireless access-point router and two PCI-card for each PC (one for the Win98 and the Win2K) right? Since it's wireless, will those PCI-card contain a cable or phone jack?

If you want to go on the internet as well, yes that is what you need. The PCI-cards have only a small antenna on the outside, no cables.

When looking for a router, check what you need first. You WILL want a firewall, NAT and VPN in that router, as well as at least 4 ethernet 10/100 connections. You MAY want a wireless access point in it, although an external one gives you more freedom if wireless standards change.
Some routers come with a serial port, to which you could connect your old 56K modem, in case broadband craps out. Some come with a parallel port, to which you could hook up your printer (thus working as a print-server). That way the PC with the printer need not be on for the printer to work.

Look around for a reliable product, check this website's forum for problems/good things with broadband.
http://www.dslreports.com/

And when you finally buy, get everything from the same manufacturer.

Is it a MUST to have a wireless router? if I want just two computers connected together, with file sharing and gaming funtionalities, but NOT going online.

Another cheaper way since you are getting a router is to get 2 10/100 Fast ethernet pci cards and two pieces of RJ45 ethernet cable plus a router with multiple ports.