Steveaudio's advice on finding 805's used is the best for your upgrade in sound, followed by amp and preamp upgrade. After securing the Matrix 805 upgrade, which I've seen as low as $600/pair used, i'd consider getting at least another outboard amp, then a dedicated pre/pro ultimately...even by the likes of Outlaw audio.
Forget receivers! Yes, upgrading your speakers will NO DOUBT be the best sonic improvment overall. (you'll run em as "small" REGARDLESS, on your pre/receiver anyway for movies at the very very least). After that you really need to consider separates in the near future. Receivers are sonically limited to higher noise level, less channel separation, less refinement, less dynamics(even in the preamp section), etc, etc. Basically, higher end refinement of sound comes from quality separates. And quality power improves even more!
Receivers only hope for qulaity is usually a digital connnection, running speakers as "small", connected dirrectly from a DVD player and such. Still, even here most dedicated pre pro's walk all over what a receiver can do...again, even something like used Acurus Act 3 and Outlaw audio 950.
I'd stay with a receiver as a budget budget only option. Then, I'd be looking for a separates upgrade as soon as possible.
Still, yeah, as an immediate upgrade I the 805 recommendation is as solid as they come for your next step.
Good luck
