Computer won’t boot up, no signal to monitor

Hi All,
When initially booting up my computer, it had indicated that I have 512MB of RAM, recently when I boot up, it reads only 256MB of RAM, this is perceptibly followed by reduced performance.
So yesterday I open her up, and pull the two 256MB RAM cards out of their slots, but put them back in the slots again, vacuum the CPU fan and Heatsink, and boot up. After booting up, it again reads 256MB of RAM.

I try trouble shooting by performing the following. Card 1 in Slot 1, Card 2 in Slot 1, Card 1 in Slot 2, Card 2 in Slot 2, and finally placing the cards where they originally were; with the following results, each and every time; The red and green lights on the tower remain solid, my computer does not make the usual beeping sound as it tries to boot, and there is no signal going to my monitor.
I have tripled checked all of the connections (video card included), and did not notice any unusual burnt smells, abnormal sounds ect.
Where do I go from here?
Many Thanks,

 

Looks like the problem is solved. For the 'umpth' time, I removed and re-installed the memory sticks and for some reason or another, everything is back to normal reading 512MB RAM as it should.
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