E-MU make some excellent audio products which are in the cheaper to medium price range.
Click on the images below for E-MU's specification for each product.
The E-MU 0404 PCIe is a 2 channel mono (can be used as a single stereo pair) PCIe card with 24 bit/192KHz converters. It fits neatly into one of the motherboard PCIe slots and has two connectors on its backplane which allow direct connection of interface cables (supplied) for the analogue line inputs and outputs, along with MIDI In and MIDI Out which is supplied integral to the device.
Going up a little in price-range is the E-MU 1212M PCIe which is an excellent product for 2 channel use. The entire interface consists of the E-MU 1010 PCIe card and the E-MU 0202 I/O daughter card, so that's two cards (worth bearing in mind when ordering a case) and will require one 'live' PCIe slot and one blank backplate position. The 1212M PCIe also has 24 bit/192KHz converters and features a MIDI interface and a Firewire port for maximum integration. Connectors are 'hard' mounted on the two backplanes and are industry standard, using 1/4" jacks for line-in and line-out.
The E-MU 1616M PCIe offers similar features for VTPO work, with the 1616M having Mastering Grade 24 bit/192KHz converters and overall better noise figures copmpared to the older 1616. It consist of a PCIe card with some connectors on the backplane including one EDI socket for connection of the outboard E-MU MicroDock/MicroDockM interface which carries the main I/O connectors and controls. They feature 4 x 1/4" balanced inputs and (what is more important to us) 6 x 1/4" balanced outputs as well as two sets of MIDI I/O, and various other useful goodies, designed to make life slightly more complicated but more comfortable! According to E-MU's product overview, the MicroDock breakout box offers a total of 16 inputs and outputs plus 32 channels of MIDI I/O.
For those on a limited budget, the older models (those without the PCIe suffix) which require PCI slots (which are becoming more rare) are sometimes available as used items, either from distributors with old stock, or from sites such as eBay and Craigslist.