Increasing memory: Which should I buy so it works with current memory?

I'm contemplating increasing my memory from 4 gb to 6 gb of DDR2-800 RAM and want to know what type and specs on the memory I should buy to add so it all plays nice together and does not decrease performance by upgrading? The system is an HP that can handle up to 8 gb RAM. My system is currently Vista x64, Q6700 2.66 ghz processor and the 4 gb of memory consists of 4 x 1 gb modules. My thought is to leave 2 x 1gb modules and add 2 x 2gb modules to give me 6gb total. Here are the details on the existing memory taken from PC Wizard 2008.

General Information :
DIMM0 (BANK0 ) : 1024 MB - DIMM
DIMM1 (BANK1 ) : 1024 MB - DIMM
DIMM2 (BANK2 ) : 1024 MB - DIMM
DIMM3 (BANK3 ) : 1024 MB - DIMM

Information SPD EEPROM (DIMM0) :
Manufacturer : MOSEL
Part Number : V916765K24QCFW-G6
Serial Number : 02A72A8C
Type : DDR2-SDRAM PC2-6400 (399 MHz) - [DDR2-800]
Format : Regular UDIMM (133.35 x 3)
Size : 1024 MB (2 ranks, 4 banks)
Module Buffered : No
Module Registered : No
Module SLi Ready (EPP) : No
Width : 64-bit
Error Correction Capability : No
Max. Burst Length : 8
Refresh : Reduced (.5x)7.8 µs, Self Refresh
Voltage : SSTL 1.8v
Prefetch Buffer : 4-bit
Manufacture : Week 17 of 2008
Supported Frequencies : 333 MHz, 400 MHz
CAS Latency (tCL) : 4 clocks @333 MHz, 5 clocks @400 MHz
RAS to CAS (tRCD) : 5 clocks @333 MHz, 6 clocks @400 MHz
RAS Precharge (tRP) : 5 clocks @333 MHz, 6 clocks @400 MHz
Cycle Time (tRAS) : 15 clocks @333 MHz, 18 clocks @400 MHz
Min TRC : 20 clocks @333 MHz, 24 clocks @400 MHz

It's 4-5-5-15 @ DDR2 667 and 5-6-6-18 @ DDR2 800 Download and run CPUZ and click the memory tab to see what it is currently running at.

Any suggestions on this last question?


Thanks