I have a 7750 AMD cpu.
Currently have 1 x 2gb of 667 Ram
Should I add 1 x 2gb of 800 or 2 x 1gb 800?
Will it access the 800 first if I put them in the correct slots on my mobo? Mobo has 4 Dimms.
Of varied speed differences of installed Ram
A computer will always work off the slowest speed only
Therefore always use the same speed Ram (and ideally the fastest available)
Go to your Motherboard or computer's manufacture support page, to find out what the maximum Ram speed is available for your system
i would get a 4GB kit (2GBx2GB) of ddr2 800 ram. costs like $45 on newegg. I wouldn't advise mixing ram with different speeds and brands can cause instability in some cases always match the ram with the same brand and speed.
Just so you know, if this is a WXP machine, you will be able to access only about 3.0 or 3.4 or 3.5 GB of the available memory... depending on the motherboard setup.
But for the price 4gb is much better deal and you still have over 3gb ram so everyone is happy. Note you can edit XP to see the 4GB but it will use about 3.5gb at most but still a step up from what you have man.