PC Memory – Can Different RAM Modules Be Mixed?

The DDR2 RAM can be run at lower frequency without any issue. The motherboard will run the RAM at 667MHz instead of 800MHz and everything will be fine. The disadvantage is that you lose a bit of performance from the lower frequency.

According to these specs here

The Max is 8GB so that would be 2@4GB Sticks

Memory 4GB (2 x 2GB) DDR3 800 SDRAM / DDR2 667 SDRAM
Total slots : 2
Max : 8GB (2 x 4GB)

So if you want to leave the 2GB stick untouched, you could get a 4GB DDR3 and have 6GB all together.

So based on you info you supplied:

However, the OEM Guide (Acer's guide for 5738/5338 users) lists the following under "System Memory":

• Dual-Channel SDRAM support
• Up to 2 GB of DDR3 1066 MHz memory, upgradeable to 4 GB using two soDIMM modules*
• Up to 4 GB of DDR3 1066 MHz memory, upgradeable to 8 GB using two soDIMM modules* Note: * Only for certain models. Note: The specifications listed above are for reference only. The exact configuration of your PC depends on the model purchased.

EDIT: I found this out from here: Link (CNET Website / PDF Document)

System memory:

• Dual-channel DDR3 SDRAM support
  
• Up to 4 GB of DDR3 1066 MHz memory,
  upgradeable to 8 GB using two soDIMM modules (for 64-bit OS)
  
• Up to 2 GB of DDR3 1066 MHz memory,
  upgradeable to 4 GB using two soDIMM modules (for 32-bit OS)
  

EDIT: After Reviewing the Model (Part) numbers of your installed memory, they are 1066Mhz modules, but your system is reporting 533Mhz, which usually is cause by errors in one or both of the memory modules, therefore the system slows the clock speed down till either no or little errors are reported. Knowing which one has the error would take running memory test.
You can download the Ultimate Boot CD and run a tool called Memtest86 or Memtest86+ to see if it will find which one this is, it will stress test and see if it can find errors.

Also did you know there is a recall on these (Link Here)