if you can go internal you're better off for small bucks.
for around £50 you can get the M audio 2496. its PCI, has 1 stereo in/out on rca (red and white!) which means its easy to hook up to a hi fi amp. And it has excellent drivers, so it runs smoothly for most sequencers. (can only talk about sonar and cubase from experience, but have never heard bad words about them from FL users.)
if you must have external, prepare to pay bigger bucks for less quality imho.
Echo Audiofire 2 is a decent firewire interface, with balanced I/O and mid, at around £100, and for USB the Tapco Link is the most decent looking box for the same price.
personally,my vote goes to the PCI card, as its far more stable, and half the price.
if you really want stuff like balanced I/O, midi, and digital I/O, try the M audio 1010Lt, just over £110.
Take a look at the EMU 1x1x series. I have the 1212m and so far i've liked it. It's got ADAT I/O (8 channels digital), s/pdif I/O (2 ch), 2 analog I/O. I think their converters are top class.
Plus you get a DSP chip in there that can do a multitude of processing without touching your CPU.
edit: oh just spotted you dont need all that fancy input output chaff. Then you could settle for a emu 0404. it's still got those Extra Special Converters and analog input and output, spdif (for example if you use MD) and MIDI. plus its dead cheap.
To be perfectly honest, these to cards are so similar that it comes down to connections: if you have a hifi amp that accepts rca go for the M-Audio and if you have active monitors or mixer or something with a 1/4" jack connection, go for the 0404. Ofcourse there are adapters and shit if you insist on going one way or the other, but i doubt you'd notice the difference soundwise. I know i didn't, had an USB m-audio thing on loan from a mate for a while.
zero problems, i've now had it for 2 years. Though i'm using windows XP. Supposedly the Vista drivers can be a pain in the ass. A valid point though, one that i would never have thought of. I guess I'll be upgrading to vista end of this year so it remains to be seen.