Have 5.1, worth adding 2 speakers or buying all new speakers?

[Slec]

This will probably be lost with the GREAT NEWS but I'll try anyway.

I bought an Onkyo HTS-650 a few years back. This is a 100wx5 set and I'm very happy with its performance, but rec'd a Onkyo TX-SR605 for X-mas. I'd like to move to 7.1 and up until today was going to buy the Onkyo SKS-540 or 750. From that, you can gather that I don't have a great deal to spend.

The Left and Right fronts are larger than the surround speakers. I know most Blu's are 5.1 but i'd like to take care of this and still have great matched sound. I see my options as:

1) buy a used system and store the speakers/pass on the duplicate receiver
2) add 2 speakers from onkyo or another brand (how should I then place the current speakers)
3) scratch this and go back to original plan of buying 540 or 750.

Any thoughts or suggestions you guys could give would be greatly appreciated?

Thanks