Need Help on upgrading RAM!!1 help anybody

PROBLEM: OK well so i got an Hp Pavilion a1719n and i have a full set of 1024 mb of ram (1GB total) i would like to upgrade and buy two sticks of 1 GB each
My specification are HERE
and my motherboard specification are here
i already know i need a 240 pin ddr2 PC2-4200 MB/sec SDRAM, non ecc unbuffered

What kind of memory can i buy a low density or high density?
which one would an expert recommend?
my a1719n desktop came with 2x512 MB already install
my system is a 32 bit
my motherboard has 4 ram sockets available two or which already have the 512 MB already on which leave two empty ones
so i would like to get 2 sticks of 1Gb each so i can place them on the two empty slot on my motherboard so i can get a total of a full 3GB ram on my system
i don't know if high or low denstiy can give me a full 3 Gb of ram after i install the 2 sticks or 1 GB each would it be hig or low density ram? because i heard that it was either low or high density ram that doesn't address the full set of memory. i want to buy the one that does addres the full set of memory,

Thanks guys.
sorry for my grammar and spelling errors

It would help to know the timings of the current memory if you intend on keeping it installed. For that, use CPUID - System & hardware benchmark, monitoring, reporting

Please post back with an image showing the contents of the SPD tab. With that information, we should be able to find suitable memory.

i would like to keep my Installed RAM
here is the picture it shows up as pc2-4300 on the icture bu t on the Ram Stick it says pc2_4200