If you use analog outputs, you will be able to get 5.1 even from onboard sound. If you want to use digital output, you want a card with support for dts.connect/interactive or dolby live/dolby home theatre.
The cheapest card to offer AC3 encoding is the SoundBlaster Recon 3D, which you should be able to find around $50. If you require a DTS solution, the Xonar DSX has DTS Interactive support and costs only a little more. The DGX is not a good choice, if you want to use the digital output.
All things considered, you might want to save the money now, and just get a mainboard with an encoder the next time.
You can use the analog output until then.