Cooling Questions

Hi
I have a Dell Dimension 8300 on which I want to improve the cooling. I want
to do this because I know the PC overheats after about 1 hr and is quite
unstable. There is currently the fan in the PSU and a fan on the back which
is vented to act as the CPU fan. I also added a PCI fan to cool the VGA
card. I can't add a fan which goes into a 5.25" bay because I don't have a
spare bay, and I can't find any other pre-cut vents for fan addition. I
think a fan sucking air in would help a lot, but there isn't really anywhere
to put one. Does anybody have any ideas or advice? For example, would
changing the HS on the northbridge chip to a HS/fan be useful?
Thanks
Peter Cavan