vista lock up problems

I have a HP media centre M7775uk-a pc, I have upgraded with 4gb Geil 667mhz
memory, plus replaced the pre-installed 8mb cache seagate hard drive with a
16mb cache seagate barracuda and also added a sapphire radeon X1950pro 512mb
graphics card due to low memory & lock up problems, if I have been accessing
a lot of info on the web, explorer will lock up and not run, after shutting
down explorer I try other windows programs e.g. control panel, internet
properties, a lot of the time nothing will run or I will get a blank page, I
have had to shut down completely & reboot to restore, occasionaly task
manager will open during this, my cached memory is 2858, which is what it is
always & physical memory is bewtwwn 27 & 35%. the problem report & solutions
page said fault in vista and I needed KB38979 performance update, but this
has already installed via windows update, I have also added updates that did
not install automatically for FSX and other games related issues.
does anyone know what going on here, I have ran windows mem test, and ran
full tests on my hardware with Pc Doctor program, everything checks out ok,
it must be software related.
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anubis116