Hi Katie's Mum,
The Linksys is a great router, you won't go far wrong with it - I've got one myself (WRT54GL as opposed to the Speedbooster one).
You're right in that you'll need something to plug-in to your PC that enables it to "talk" to the wireless router... this is commonly referred to as a Network Interface Card, or NIC for short. You're also right that there are 2 types - at least for a desktop PC... there's a different one specifically for laptops, but I'm assuming your PC is a desktop one. These are, as you already know, the PCI and the USB variants.
There are a couple of technical differences between the 2 which make the PCI version a better choice than the USB one, but I wouldn't worry too much about that. Instead, what's probably going to be more important for you, at least at this stage, is that the USB NIC is an external device that you just plug-in to a USB socket on your PC... and you're finished. The PCI version, however, is an internal device, so you'd need to take the case off your PC, slot the PCI NIC into a PCI socket on the motherboard of your PC, and replace the case. It's not difficult at all - it's a 5 minute job at most, however, I realise that you might not want to go installing cards to your PC, in which case, the USB version is much, much easier to do.
From reading your post, one thing you should be aware of, however, is that you're going to need to take the PC downstairs once and position it near where the wireless router will be situated (which will be next to where your cable modem will be installed). This is because you will need to configure the wireless router the 1st time you switch it on, but you can't do this wireless-ly - you need to use a network cable that will be provided with the wireless router. This will plug-in to the back of your PC... most modern PCs have a network card built-in to the motherboard, you should see one on the back of the PC that looks slightly bigger than the socket you're using for your dial-up modem.
The instructions on the wireless router will explain exactly what you need to do to set it up so you can connect via wireless, so just follow those. In fact, the Linksys routers also have an application you can download which will automagically configure the router for you.
Once you've done this, your next step will be to think about securing your wireless connection to stop anyone else connecting to it... but we'll leave that for another thread for now
If you can follow flat-pack instructions then you'll find installing and configuring a wireless router to be an absolute walk in the park
