tsnor :
I'm surprised by "Stay away from Quads, because you will have to dramatically upgrade your cooling and your power supply is not strong enough for them,"
Power draw of E8400 is 65W
http://ark.intel.com/products/33910
Power draw of Q8400 (highest wattage quad on the list) is 95W.
http://ark.intel.com/products/38512
An extra 30W is not going to kill his case temps or PSU.
Unfortunately I am right:
Celeron 450 is
35 watt (this is what OP has now)
http://www.cpu-world.com/CPUs/Celeron/Intel-Celeron%20450%20-%20HH80557RG049512.html
I provided possible upgrades for motherboard (Mini Tower and Slim share the same motherboard), so it is possible, but look below for the explanations.
Now about CPU coolers. You think the are all the same? Wrong, they are different, Dell tried to save every penny here.
I could not find Celeron cooler picture, but I seen one in actual PC (not on the picture), it is smaller or very close in size than first picture below
C957N Heatsink & Fan Assembly for
65W CPUs
C955N Heatsink & Fan Assembly for
95W CPUs
It is known that upgrade from 65 watt CPU to 95 watt CPU requires cooler upgrade - many people complained about overheat in Mini Towers, Slim version even smaller, so you see.... And now OP has to jump from
35 watt to 95 watt on the same cooler, humor me....
I had same cooler (65 watt) on my Inspiron 570 before and I did extensive research in this area, so prove me wrong... (no offense please).
I know that people were making extra holes with drill on the side panel for extra ventilation and so on - this all combined makes cooling upgrade a huge headache! Increase from 35 watt to 65 watt will require only cooler upgrade, no extra fans necessary, so this is minor upgrade.
Also, you can not use after market CPU coolers so easy - look at the motherboard connectors, those are screws for LGA775 socket (how do I know?).
So, if OP is ready for the challenge - go for it, just be ready for serious cooler upgrade and maybe some improvisation, add 7750 to the mix and you are have 50/50 chance of burning your low power CPU, it can be upgraded using TFX Seasonic (350 watt), but this is extra money...
Anything else I can help you with?