Replacing front pair of PC speakers

A few years back I bought myself an Onkyo HT-S5105 HTIB and it has stood me in good stead so far but I thought I would see if I could improve things a little. I hook up my PC to my Receiver with an optical cable via my Creative X-Fi Fatal1ty Pro PCI-E soundcard.

I was thinking about spending up to £50 (make it easier, no more than $100 USD) on replacing the 2 front speakers and replace the rear speakers with the old front speakers. That way I hope to receive a reasonable upgrade to both my front and rear speaker sets.

My computer is mainly for gaming but I do the occasional movie and music thing with it so I'd prefer something to enhance the gaming side of things mostly.

Current Setup:
Onkyo HT-R518 Receiver (Manual)
Onkyo SKW-518 Subwoofer
Onkyo SKC-518C Centre Speaker
Onkyo SKF-518F Front Left and Front Right Speakers
Onkyo SKM-518S Surround Left and Surround Right Speakers

Is there much that can be done with my budget? I know that some speakers and speaker sets easily run into hundreds and thousands of £/$'s but I'm working on a tighter budget . I know I cannot expect a huge difference in the quality of my front speakers but I was hoping that I could get a nice boost there as well as a substantial boost by moving my current front speakers to the rear.

I'm still very new to things and as I read it, my receiver can accept any speakers with an impedance of between 8 and 16 ohms.

Any support welcome

EDIT:Added some extra information to try and help you guys and gals help me better.

Onkyo Front Speaker Specs
Type: 2 Way Bass-reflex
Input sensitivity/impedance: 8 Ω
Maximum input power: 130 W
Output sound pressure level: 78 dB/W/m
Frequency response: 60 Hz–50 kHz
Crossover frequency: 4 kHz
Cabinet capacity: 1.8 L
Dimensions (W × H × D): 101 × 273 × 121 mm
(incl. grille and projection)
Weight: 1.1 kg
Speaker: 8 cm Cone Woofer
2.5 cm Balanced Dome
(Tweeter)
Terminal: Spring type color coded

Onkyo Rear Speaker Specs
Type: Full-Range Bass Reflex
Impedance: 8 Ω
Maximum input power: 130 W
Output sound pressure level: 79 dB/W/m
Frequency response: 70 Hz–22 kHz
Cabinet capacity: 1.1 L
Dimensions (W × H × D): 101 × 175 × 116 mm
(incl. grille and projection)
Weight: 0.7 kg
Speaker: 8 cm Cone
Terminal: Spring type color coded

 

My inexperience in these matters shows. I was only wanting to replace the Front Left and Front Right speakers. The Centre Speaker will be unchanged.

As for the Pioneer SP-BS21's, I'll run them through the UK stores and see what kind of price I come up with. Thank you for the suggestion.

I am not opposed to buying a pair of used speakers. My problem is knowing what speakers to look for at my budget level