What antivirus should I get? Avast?
Kaspersky, Trend Micro and Nod32 are probably the best paid ones.
If you don't want to pay, get Windows Security Essentials.
How big is your case?
Dont know but i know its an Acer Aspire one X1300 and its Quite compact
What slots does your motherboard have?
See Above
What wattage/model is your power supply?
See First Answer
What OS are you running?
Windows 7 64 Bit
What is your CPU?
AMD Phenom 9650 X4
That case looks really compact.
You won't be able to upgrade that graphics card much. Save for a new system or get a new case/mobo/PSU.
Would this work with a Toshiba Satellite C655D? I tried comparing specs, but the only two factors I know are that the Satellite's max Memory is 8GB and that they are both DDR3.
(The Satellite: http://www.toshibadirect.com/td/b2c/pdet.to?poid=2000014196)
Possibly.
It's hard to say without the full specifications of the RAM DIMM slots, with speed specs.
Although with that amazing (lol) 1.3GHz dual core, your RAM won't really make much difference. Try find more detailed specs on it.
Is this risk free?
Nothing is risk free. Up to you whether or not you want it fixed.