My favorite hardware mod is easily the IR power mod, as palbot said XERC, other "brands" you might consider are xIR or IR-Mod
A front panel LCD is always really cool, I especially like it's integration with XBMC. you could easily buy one of those swanky new X3 front panels with the plug and play LCD and hole pre-cut.
Despite what others have said I think that the ATA100 IDE cable upgrade is a great mod. If you'd like to know why... read this:
http://www.xboxrepai.../newsdesk_id/28Most people will knock it but those people have never been able to point me to test results that back them up. For me, if you have an ATA100 HDD it's worth the few bucks for what I consider to be a worthwhile speed increase.
You could also consider some cooling modifications, such as upgrading the thermal compound on your CPU and GPU, upgrading the exhaust fan to an 80mm, you could also add heat sinks to things like the MCPX or your RAM.