You can upgrade to the Turion MT processors, but they will run hotter than the Sempron. Upgrading the CPU can be kind of a pain since you have to open up the entire bottom and remove the hard disk to access one screw to remove the heatsink.
You can mix RAM sizes, but I would avoid it as you will get dual channel with matched pairs. It really won't do any harm, but you would be doubling the RAM bandwidth by using pairs.
And finally, I seriously doubt you will find a motherboard that will work. The best option is to upgrade your RAM for better gaming performance. You can also increase the amount of video RAM in the BIOS from 64 MB to 128 MB.
You can mix RAM sizes, but I would avoid it as you will get dual channel with matched pairs. It really won't do any harm, but you would be doubling the RAM bandwidth by using pairs.
And finally, I seriously doubt you will find a motherboard that will work. The best option is to upgrade your RAM for better gaming performance. You can also increase the amount of video RAM in the BIOS from 64 MB to 128 MB.