Mixing makes, sizes and speeds of RAM is not usually an issue. Just make sure that they are paired correctly (in the right slots.)
As already stated, the motherboard must support more than 4Gb of RAM. That's easily checked on the makers web site. A board from this era may or may not support that much RAM. It may also require a BIOS update to do so.
As also stated, a 64 bit OS, such as Win7 64, will be required. That's also easily determined by looking at the System properties.
Win7 should run fine with 4GB of RAM. I would also be hesitant about upgrading the RAM on this system. RAM is a bit pricey right now and DDR2 RAM is basically obsolete. Removing or uninstalling unnecessary resident programs is often the best way to speed up a system. If the system is short on usable RAM, it may be better to tune the OS by determining what programs are using the most RAM and disabling or uninstalling those that are not needed.