I have Dell vostro laptop which has 2 GB RAM. It has one more slot where I can add 2 GB more so it will become 4 GB. I want to use my laptop at maximum possible speed. I am planning to take my laptop to computer shop to upgrade RAM. Which RAM I should buy to get maximum speed?

I checked my hardware configuration with one utility which says

Memory Summary
Maximum Capacity 4096 MBytes
Memory Slots 2
Error Correction None
DRAM Frequency 399.0 MHz
Memory Timings 6-6-6-18 (CL-RCD-RP-RAS)
Device Locator Slot 1
Manufacturer Samsung
Part Number M4 70T5663EH3-CF7
Serial Number 955A03E2
Capacity 2048 MBytes
Memory Type DDR2 (PC2-6400)
Speed 400 MHz (DDR2 800)
Supported Frequencies 266.7 MHz, 333.3 MHz, 400.0 MHz
Memory Timings 4-4-4-12-16 at 266.7 MHz, at 1.8 volts (CL-RCD-RP-RAS-RC)
Memory Timings 5-5-5-15-20 at 333.3 MHz, at 1.8 volts (CL-RCD-RP-RAS-RC)
Memory Timings 6-6-6-18-24 at 400.0 MHz, at 1.8 volts (CL-RCD-RP-RAS-RC)
Data Width 64 bits
Manufacturing Date 2009, Week 4
EPP SPD Support No
XMP SPD Support No

I can add 2 GB more RAM and I am planning to buy this weekend.

Can I buy DDR3 RAM for this laptop with different frequency ? Is it possible for laptop to have one 2 GB module DDR2 with 399 Mhz and another 2 GB DDR3 with higher frequency?