I
believe density was a marketing term used on conjunction with larger single memory sticks. Basically, as areal density of the individual flash memory IC's increases the total capacity of the stick increases as well. So you could have 8 IC's of low density memory on one single sided stick of memory and have it be say... 64 MB total (8x8MB), or you could have 8 IC's of high density memory and have it be 256 MB total(8x32MB). The numbers don't matter, its just a fancy and unnecessary term to describe modern memory technology. To sum up, -old- memory was low density because we lacked the technology to pack in more transistors, thus resulting in lower capacities. New memory, like in the last decade is considered -high- density because we chose to use that term. We can now pack in more transistors than we could before obviously. Thats why now you can buy a single stick with 2 Gigabytes on it, which is made up of 8x256MB IC's.
I might be wrong on some of this, I had to do some quick research to fill in the blanks in my own knowledge to explain it.
As far as High/Low Density compatibility I'm not sure where you heard that from. All memory you purchase in todays world is -high- density, just find the speed and brand of memory that you like and buy it. If you make an educated purchase the chances of you buying something that you cant use are very very low. Be aware of memory type(SD, RD, DDR, DDR-2), Speed (PC2100, PC3200, etc), ECC (Error Correcting Code) and if it matters to you personally brand and latencies.
Its a Socket AM2 board, with a X2 chip if I'm not mistaken that combination requires DDR-2 800 memory, also known as PC2-6400.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Description=DDR2+800&x=0&y=0