Mucky,
mah bruddah. That new electronics smell is what modern man uses to replace the thrill of the hunt, or traveling to neighboring villages and stealing their wimmen. Buying a new slimline 360 is a surrogate for fulfilling those primal urges. So if you don't have a neighboring village with lax laws about kidnapping, you'll need to buy the new box.
I'm so on the fence about this. Here's what I've got:
refurbed 360; unsure about which rev
/g/b wifi adapter
120GB HDD
Here is exactly what would improve:
HDMI,
/n instead of /g wifi,
130GB more storage, and it'd be
quiet.
Probably slightly lower power usage, but no-one's giving "eco" discounts for game hardware, only fridges, ovens, and air conditioners.

I could drop 150USD on an Arcade unit to get the HDMI, or 300USD to get the "whole package" but neither option is a substantial improvement over my already well-accessorized 360.