Do you already have any other A/V equipment, such as a DVD player, PS3, etc.?
You can put together a system for under $800, and you may get several suggestions here to get you started on your search. With your budget, I'd normally recommend a budget A/V receiver such as the Onkyo 506 that sells for under $200 on sale. Onkyo's 500-series receivers have gotten good reviews for the past few years. Your speakers have an impedance (and I'm guessing that's nominal) of 4 ohms, which is below the general guideline for the 506. However, the 506 manual just says that the amplifier's protection circuit might activate if you play speakers with an impedance below 6 ohms at elevated volumes for an extended time. So, hopefully you would be okay - but it is something to consider.
Matching the rear speakers to those Snell's isn't critical, and you could even go with some fairly inexpensive rears to start with just to get some sound back there. However, it would be best for your center channel to match them. I'd suggest contacting Snell to inquire about that. They might be able to help you out, but they might just point you towards their line of center channels. You can always run the surround sound with a "phantom" center, which just means that your front speakers mimic the center channel.
As for a sub, there are a lot of options out there. A brand that I've never heard, but have seen a number of recommendations for on this forum in your price range, is Dayton. Partsexpress.com sells those subs. I'm guessing that you'll be looking in the $200-$300 range for a sub, once you get a receiver, speakers, and cables. Speaking of cables, you don't need expensive ones - just browse around this forum for some ideas...or ask here if you want.
If you also need a DVD player, that will cut into your budget by around $100.
Adam