Scorch Marks?

[A Marshall]

I'm begining to notice what appears to be a slight burning around the outlet on the back of my cube (above the graphics card).  It's only slight, and rubs off with my finger leaving a light brown deposit on my finger, this is both on the cube casing & the metal mesh.  I'm currently running a GeForce 2 card pulled from a tower (3 or 4 months or so) before i was using the original stock Radeon (but pulled it as fan was getting noisy).  Is this normall over time? (my cube is about 17 months old) it does get rather hot at the back, but i wouldn't say it was any hotter with the GF2 (if anything the radeon seemed hotter).  

At the moment i was thinking it may be due to dust deposits? (which could be making things apear worse), my girlfriend smokes like a trooper & is always smoking near my Cube (no matter how many times i tell her...) could this play a part in it?, or should i be thinking of getting a fan installed ASAP?

Regards,
'a