This is probably the single most asked question, and my answer hasn't changed over the years.
I'd say your system easily meets the needs of video editing. To be honest, your best bet is to buy a bog standard Firewire card if you've got a DV camera. Look at the "bundled software" that come with it too - capture cards at the low end of the market are purely seperated by the bundled software (all standard Firewire cards are essentially the same).
Take a look at the software available, choose what you want, then go for a card that bundles this software. Examples of bundled software are VideoStudio, Pinacle Studio, Power Producer and EditStudio. If you already have a software package, just go for the cheapest card you can find.
If, however, you want to have the added benefits of hardware accelerated encoding and rendering, you should expect to pay around about half the price of a standard PC on the card alone. Packages bundled with these cards would include Adobe Premiere - which retail at approximately the same price as the card (so you really do get these for free!)
Finally, if you're planning on doing any analogue capture, you'll want a card with analogue inputs, which again pushes up the price when compared to a standard FireWire card.
Essentially, you can spend as little or as much on a capture card as you want. But my advice for beginners is to buy yourself a cheap Firewire card use a trial version or freeware apps until you're sure you're going to get the full use of more expensive equipment. If you do catch the bug, upgrade to the full version of the software (it'll be the same price as if you'd bought a bundled package anyway, so you won't lose out on anything), or invest in a mid range video editing card from a manufacturer such as Canopus.