Upgrading my corrunt system

Hi, Im new to this forum, Im Aaron from Israel.
So like this...I've been to the US where I bought a pretty decent system for my needs from a garage sale..lol..It includes 5 sony speakers which are:
Front: SS-MF315
Center: SS-CN290
Surround Back: SS-SR290
And all that for 35 dollars, and they still run great. They're connected to a Yamaha HTR-5750 Reciever. So everything is VERY expensive over here and Im only 17 so there isn't a big budget but I thought of buying a used AM5 Bose system to replace the front speakers. What are you're suggestions? it's either that or buying a powered infinity subwoofer which I don't know the model for but it sounds great and I only have a small room. I compared it to our polk audio sub in the living room both have very deep and warm bass...So guys...what should I do?
Thanks in advance!~
Aaron

ok...I guess for now I will stay with the same speakers and go for the subwoofer addition. Now I mostly listen to the computer through the system for movies and music although I have virtually everything connected to it, DVD, PS2, Cable, CD Player and PC. So right now I have a Creative Sound blaster Live 5.1 and the pc is connected to the reciver by an analog connection. I thought about buying the new card which is X-Fi Xtreme Music which supports a Digital out (SPDIF out) and then connect it digitaly. Will it make that much of a difference in sound quality? Is it worth buying?