Should not be a problem. According to the Intel site, the processor can handle 1066 or 1333, and up to 8gb. However, it does work a little better with dual channel(both slots filled). If your intent is to get one 4gb module now, and another later, that will be fine. I am assuming it has 64 bit Windows 7 on it? If so, 8gb ram(2x4gb) would be ideal. 32 bit Windows will not recognize or use above 4gb.
It sounds like a very nice machine.