1) I heard that windows 7 is installed into the original motherboard, so if i change it, it will boot up without an OS?
2) Will the new motherboard fit in the desktop?
I realize i have to get DDR3 ram and most likely a new PSU.
Thanks a lot.
Yes you can replace MB. Most people also get a new case because the OEM case may only have holes for the OEM MB, the OEM wires (audio front, power, etc.) are typically not labelled so it takes more effort to match them up, etc. Good new case is not cheap = $50 to $100.
OEM win7 is locked to the MB, but microsoft will let you re-validate windows. Go to Microsoft support and search for re-validate windows procedure. Ignore the people who say you can't. I've re-validated a few copies of windows, but don't trust me, read the procedure. It's simple.
Motherboards come in several standard sizes. Your pc's spec will tell you what size. probably micro ATX, but find out. Note you have no guarantee that a new MB will fit in an OEM case if it's not the oem's board. However most likely it will.
____ BUT IF YOU WANT GOOD GAMING DON'T DO ABOVE______
If you want a stronger gaming PC then you have enough CPU -- you just need better video.
Your AMD Phenom II X4 810 2.6GHz Processor + MB + Mem = pretty good
Your ATI Radeon HD 5450 Graphics = not so strong.
6.0GB DRAM = plenty
Power 300 Watt
You can stuff an AMD HD7770 (80W card) in your case instead of your HD5450 (20W card used mainly for home theater, not gaming) and keep your existing 300w PSU for $125. You'll be shocked by how well your system can game.