Acer Aspire E360 - add memory

I have Acer Aspire E360 with AMD64 3500 on a Foxconn MB. I'm trying to double the RAM, but not buy expensive Acer OEM.

It has 1 Gig DDR RAM: Nanya brand, a pair of 512, double sided. On the chip carrier it says NT512 D64 S8 COG-5T/512MB DDR-400-CL3/PC3200U-30331.

I tried to add more RAM. First I purchased Corsair XMS, but it slowed my system to a crawl (some Java compiles literally ran 10x slower!). Corsair tech support said I had to change memory settings in the BIOS - but these are not available in this BIOS.

So then Corsair said I needed Value Select memory that runs on 2.5V (XMS memory runs on 2.75). So I got this and it was even slower! I noted with interest that this memory was single-sided. I experimented by removing the original RAM and only using Corsair - this worked fine. Swapping their positions did not work well. So basically Corsair said "sorry Charlie", buy OEM RAM.

Any ideas?

Here's some system info:
System Manufacturer Acer
System Model Aspire E360
System Type X86-based PC
Processor x86 Family 15 Model 47 Stepping 2 AuthenticAMD ~2210 Mhz
BIOS Version/Date Phoenix Technologies, LTD R01-A4, 10/28/2005
SMBIOS Version 2.2