Best Computer Speakers - Need Help

I'm an avid computer user. I have a dual 19" LCD monitor display, a soundblaster Audigy 2, an ATI All-In-Wonder TV tuner card, and currently a Cambridge Soundworks 215 - 8" subwoofer with integrated BASH amp for the speakers (60 watts x 2) and sub (150 watts) using Cambridge Soundworks Newton M50 speakers for a total of 270 watts. I listen to DVD-audio, CD audio, TV, radio, and mp3's alike on my computer. I have the speakers hooked up to the sub-amp using some cheap Monster XPMS wire with Monster Quick Lock Flexipin connectors. The main limitation of the speakers is that my wire connecting the source to the sub-amp is just a standard 1/8" miniplug to my soundcard.

My biggest problem, right now, is the setup of my speakers. The sub is underneath my desk by my feet and completely surrounded by the 3 walls of my desk (3 feet width, 2 feet depth, and 2 1/2 feet height). The speakers are placed in a horrible location on top of the bookshelf on my desk. The speakers (when sitting down) sit at the height of the top of my head. The left speaker is about centered to my head, wile the right speaker is about 2 feet to the right of the left speaker.

I'm on a quest to get the best possible sound from my computer within reason. I've actually debated upgrading the 5 1/4" woofered 2-way M50's to Ascend Acoustic CBM-170's. My main reasoning is that I've heard the 6 1/2" woofer of the CBM-170's can accomodate the bulk of the music (down to the lowest note of a bass guitar) without reverting to my rather weak and ill-placed sub.

So my main question is can a 6 1/2" woofered bookshelf vs. a 5 1/4" woofered bookshelf speaker make a considerable difference? And would it even be worth upgrading to the CBM-170's considering all the above-mentioned weaknesses of the rest of the system?