Noise and buffer sizes

[Gregjohn]

I’m no newcomer to computer music recording and have overcome most of the common problems over time but these two have foxed me:

1) I seem unable to reduce the buffer size to less than 256 without much crackling. (I'm using edrums to control softdrums here and don't want to have to move tracks around to get them back in time)
2) Even when I have no applications running there’s more hum than there should be coming from my speakers. Added to that if I move the mouse there’s a crackle that corresponds with movement.
Admittedly the noise is only apparent with the amp at fairly high volume.

The equipment and software?

Windows Xp service pack 3,
Cubase Sx3,
Intel Core 2 Quad CPU Q8200 @2.33Ghz, 4GB of ram,
Nvidia 7100/Nvidia nForce 630i (onboard),
Alesis iO26 Firewire interface (external soundcard)
Generic PCI firewire card.
Western digital caviar 500gb sata drive
Pioneer dvd writer (sata)

No other unnecessary software other than fx or softsynths.

I’ve checked for earth loops, tried different mouse/keyboard combinations, and updated all drivers, run every cleanup and registry checking software I possess. I disabled the onboard sound in the bios. The network cable is disconnected unless I need to update anything.

Interestingly there is no noise when monitoring on headphones plugged into the Alesis iO26.
Also if I unplug it from my amp (and speakers) there is no noise from the amp.
So it appears that the noise comes from the output sockets only.

Any suggestions ideas appreciated.

Oh yeah I should have said hi first Hi!!

Greg