Since we don't know what you currently have, it's hard to say what you would consider a huge upgrade or not.
One thing I can say is that I remember having my entry level, bottom of the line Kenwood home theater in a box A/V receiver when I got my Definitive Technology BP30 speakers, and the difference in audio quality was night and day. To the point where I almost didn't want to upgrade my receiver anymore.
Good speakers make a huge difference, and the price you pay for them typically is in line with the quality you get. But, you should realize that many people here may have single speakers costing $1,000 each. So, you really will want to determine what level of quality you are prepared to buy into.
I would agree though, you can find some good deals on eBay and Audiogon if you are willing to spend some time looking.
Likewise, discontinued receivers are popping up at great prices over and over. If 3D is not vital, then you can pick up a refurbished Denon for just a few hundred dollars which was $1,000 or more just last year.
So, something like this may be a great deal (I haven't researched the pricing)...
http://www.audiogon.com/cgi-bin/cls.pl?spkrfull&1311307649&/Definitive-Technology-BP-20-bi
As some people upgrade, and others downgrade and just want to move their stuff out of their homes.
Subwoofers are tricky. I've used the Epik product and others, and at the end of the day, I will swear by a sealed enclosure subwoofer with lots of power behind it. Epik sounds great. For smaller packages, Sunfire and Velodyne DD (or older HGS) series sound great as well. But, it depends on the size of your room.
There are a ton of speakers on eBay, so finding the right product at the right price is just a lot of homework on your part, but deals are there.
I would add that receivers are tricky. Expensive receivers are VERY quickly outdated, while cheaper ones almost never have good power behind them. I like Pioneer, but they've always seemed a bit underpowered to me when they are driving a full 7.1 setup on their lesser expensive models. Of course, that may hold true for many receivers. They do have solid quality which I can recommend though!