I've yet to use the mouse included with my G74SX-A1. Did you confirm the touchpad was disabled when you tried yours?
What version touchpad driver are you currently running?
If the spacebar issue hinders your typing, I would recommend exchanging it for a new unit, if possible. Alternatively, this thread (
http://rog.asus.com/forum/showthread.php?11919-Asus-g74sx-Spacebar-issue) discusses how one user modified his spacebar key with a shim. Perhaps you've seen it?
The G74SX BIOS, ATK Package version, DPC Latency, Browser and Keyboard configuration can also contribute to missed keystrokes and keyboard lag. Here is what I did to improve keyboard performance:
http://rog.asus.com/forum/showthread...ll=1#post44735
G74SX-A1 - stock hardware - BIOS 202 - 2nd Monitor VISIO VF551XVT