Agree with Timo in relation to future products based on PCI- Express. They won't be around for a while yet while the Firewire based kit appears to be ruling the roost. In the meantime...
A good buddy of mine swears by the RME HDSP Hammerfall 24/96k card if you are looking for a quality computer based soundcard. You can probably pick them up quite cheap these days on ebay.
If you would rather go down the external Firewire/USB 2.0 unit you cannot go wrong looking at either the MOTU or RME range. Try also looking at the latest Mackie Onyx range - the 400F is getting some good press and reviews.
If you are on a budget go for the MOTU or the Mackie range. If you are flush go for the RME range.
Listed below are a few of the units you may wish to research or even demo at your local pro-audio store.
MOTU Traveler - (Firewire)
MOTU 828 MKII - (Firewire or USB 2.0) www.motu.com
RME Fireface 800 - (Firewire)
RME Multiface 2 & PCI bundle www.rme-audio.com
Mackie Onyx 400F - (Firewire) www.mackie.com
It is well worth you saving the extra bucks for such a critical piece of your studio. You can pick up most of the above on ebay very cheap second hand.
All the best.