imo... you should get an external firewire ASIO supportive unit because it brings the fastest results with the lowest latency

there are of course, many companies you could check like: E-Mu
M-Audio, Tapco, Edirol... they have some non expansive soundcards with a reasonable - high quality, each with its own advantages .
i had my experiences with the E-Mu 1616m PCI card that Stephen suggested and it is a pretty good choice, it comes with a nice PatchMix system and an uncanceled amount of built in effects, produces low latency and a good sound color.
but...it is not only the interface that does the job... you should also have a computer that is strong enough... a pair of reasonable near field monitors... you already have your talent and your two hands... and you're good to go.
i really don't know what you are using at the moment so maby you already have all that or know all about that and i just gave you a headache
