From notebookreview.com
"Do I need the same size RAM sticks to run in Dual Channel? Yes (AMD at this point says yes)/No (Intel) but you do usually want the same speed and latency as every thing clocks down to the slowest of both. Example 533Mhz CL4 mixed with 667Mhz CL5 will go with the slowest of both 533Mhz CL5 (not always with the latency) and slower than if either were matched. Same size sticks will run slightly faster symmetric RAM array (Interleaved). If the RAM is of different sizes it runs in what is called asymmetric RAM array (Intel Flex Memory/ AMD does not support). The greater the disparity between your 2 sticks the greater the performance hit vs sticks of the same size. The difference seems to be approximately in proportion to the ratio of the the smaller stick x2 to the entire amount of RAM. An example with Intel, 1GB stick and 2GB stick. 2/3, 1GB(smallest stick)x2 to 3GB(the entire amount). Well 2/3 of the total improvement, of the Dual Channel running with the same size sticks (symmetric RAM array) 10%, the asymmetric RAM array example is about 6.7% so a 3.3% less than symmetric RAM array. With 512MB stick and 2GB stick, 512MB(smallest stick)x2 to 2.5GB(the entire amount). 1/2.5 4% improvement. AMD's will show greater improvements and losses, Dual/Single keep that in mind."
Unless they came out with 3gig sticks, I'm assuming you were going 4x1 and 2x1, in which case you want to just avoid doing that as it actually bogs down performance as it goes into asymmetry. 8gigs is way too expensive for what it is right now and it's just smarter to build a decent desktop with faster 8gigs of ram for the equivalent $600. Though the portability factor could be essential, I just cannot justify $600, even in light of absolutely needing portability.
