There's lots of good deals on used stereo speakers out there in thrift store and yard sale land. I've picked up pairs of US/Canadian/UK-made speakers for between $6 and $25 a pair. Some could use new grille cloth, but they all worked fine, and originally sold for at least 10 times what I paid. The main risk is rotten foam surrounds (the rubbery ring around the woofer); some are made of a compressed foam that has good acoustical properties, but often deteriorates over time. It's not hard to replace the surrounds yourself, if it's a standard size (usually under $35 for a kit; much less if you can buy just the surrounds from a dealer in electronics service parts like Main Electronics in Vancouver). It helps to be familiar with audio brands so you can avoid the "white van" speakers.
Stereo receivers are common, too. I've seen Dolby Digital receivers from $25, and even an HDMI receiver for about $50. Some "home theater in a box" DVD/processor/amp units may be worth considering IF they have inputs for external digital sources. Power output will be modest, but generally the weak points are the teeny tiny stock speakers. They probably won't let you modify any of the speaker management settings that proper receivers or processors have, like "phantom centre" or whether to redirect bass from the main speakers to the sub (or vice versa if there's no subwoofer).
The way I see it, every dollar you don't spend on mediocre new stuff is a dollar you can put towards a nice HDMI receiver. (I'd suggest going with a 7.1 model since those tend to have other useful features like upconverting standard def inputs, so you don't have to change inputs on the TV as well as the receiver when switching between high and low definition sources).)
Keep in mind that any decent A/V receiver can be configured to use "phantom centre" if you only have 4 surround speakers, so you can enjoy full surround sound with just two pairs of stereo speakers. While front and rear should ideally match, in practise even identical speakers don't sound the same when located in different parts of the room, so don't worry about it. If you pick up a pair of bargain stereo speakers and are particularly pleased with them, get a matching pair from a classified seller.
When you're ready for a subwoofer, building it yourself is an attractive option. It's just one box that can be hidden in a corner or behind furniture, so doesn't have to tax your carpentry skills (although I highly recommend a router for best results). Take a look at the DIY section at AVSforum. Spending maybe $300 on a woofer and amp will get you something close to a $1000 retail sub. And next to a big screen, nothing makes more impact in a home theater than serious bass.
