....don't get me wrong: zimv20's setup is a great recommendation. The presonus USB box is quite good, and it will do a good job in your specific task - and the price is very nice. Some of us here are very picky about sound, so we'll pick at any small thing

. Also, zimv20 is right: really
really clean-professional-pristine, almost perfect preamps start at around $800
per channel... but trust me: in internet / flash audio, any small leak over the preamp will be barely noticeable... and you can fix it's gain via software in the final mix, specially if you have a good microphone and software to do it.
Keep in mind that when it comes to prosumer audio, and specially portable gear, it's a balancing act between budget, number and config of I/O's, portability, flexibility, sound quality, etc etc etc.... it really helps to have a budget - but most important is to stick with it. Don´t worry too much about everything - no piece of equipment is perfect, specially in the prosumer portable segment.
So,
- don't discard the USB Presonus box lightly - it's a great deal for the money. Also, Presonus makes a firewire box of which I've heard good things. Check it out.
- +1 for the Apogee duet... had one for a while - great preamp. If you are OK with limited inputs and outputs, it's a great deal in the $500 range.
- Lately I've been using a MOTU firewire as my portable interface. I really like it for its flexibility... but - as all portable interfaces - the preamps are not perfect, specially when you compare them with desktop/studio gear.
- If portability is not an issue - desktop mixers are a whole other ball game when it comes to quality, I/O's and prices. Mackie has a nice range of very good sounding desktop USB and FW mixers, from basic U420 series to more sophisticated Onyx mixers.
but getting a dedicated mic pre means you'd still need a dedicated a/d & d/a converter, and then you'd still have to address monitoring.
wrapping those 3 things up in one unit (an interface) is pretty handy.
- If you plan to strictly do professional voice overs - just 1 microphone - need great sound, a nice studio-grade tube preamp, have a mac with optical sound input (combined jack; any intel mac from 2006 to date), this Aphex 230 VP is what I use for such tasks. It's a one trick dog, but a great one for that matter. This will make your VOs sound studio-like - totally worth it (you can get it $100 cheaper w/other sellers). Check the demo video out here and here - and it'll also will clarify some terms involved... It will deliver digital sound directly to the computer, has a nice gate, compressor, de-esser, etc. Monitoring has to be done with a separate box - either a headphone amp box or a simple interface to avoid latency- the price range for this is very flexible. An Aphex230 + Shure SM7b mic package + headphones + headphone amp... still within your new budget.
IME, if the voice-overs you're doing are not yet professional (e.g. you'll get paid good money for them), I'd also consider the USB mic route for starters. A good USB mic (such as the
Rode Podcaster) is always helpful - even if you later decide to get more sophisticated gear. It'll also help you develop your microphone technique, which when it comes to VOs, it's just as important.
And if you plan to jump into a more sophisticated setup right away - XLR mic, interface, etc etc - just remember the microphone is the beginning of the chain, so try get a nice one, such as zimv20 suggested... you can´t go wrong with the Shure SM7b.... I found this great
deal with the mic, boom and cable.
Hope this helped,