Soss, the Multiface can only be used with the HDSP or HDPSe card. The difference between these is not sound, but latency. HDSPe uses a PCIe connection, which is faster than PCI, giving you lower latency. At least, this is my understanding.
The Multiface has no preamps. You'd need to run from the XLR of external preamps using an XLR to TRS cable. If you only need two channels, RME pres are probably good enough for you. I wouldn't recommend getting anything of higher quality unless your needs are such that you'd want want the quality of third party pres that comes at $1,000/channel.
If you're using VSTs that you want to hear in as close to real time as possible, PCIe is your best bet. Firewire is not as fast and USB is even worse. Once again, this is just my understanding from much internet searching. I based my purchase of the Multiface II and PCIe card on all of the above. I've only read rocking reviews all over the internet.
If you're just recording yourself and a guitar, for example, and don't need to monitor software instruments or effects through your computer, you'd be better off saving money and getting the Babyface. Then you won't need to buy the $500 sound card or additional pres.
If you think you're going to want more than two channels in the future (or the ability to monitor at extremely low latency), go with the Multiface. You can get two of these http://www.sweetwater.com/store/detail/TubeMP/ Never used them but they're said to be insanely good for that money. Search for threads on Gearslutz.com