I'm not a comp wizard but I know enough of the basics to pick RAM even when buying from the net. I have a Emachine T3516.
Intel® Celeron® D Processor 352
(3.20GHz, 533MHz FSB, 512KB L2 cache)
Operating System: Genuine Microsoft® Windows® XP Home (SP2)
Chipset: ATI Radeon® Xpress 200
Memory: 512MB DDR (1 × 512MB) 400MHz (PC3200)
Expandable to 2GB
Hard Drive: 120GB (7200rpm, 2MB cache)
Optical Drive: 48x CD-RW/DVD combo drive
Video: ATI Radeon® X200-based integrated graphics
Up to 128MB of shared video memory
PCI-Express® (PCI-E x16) slot available
Sound: 6-channel (5.1) high-definition audio
Network: 10/100Mbps integrated Ethernet LAN (RJ-45 port)
Modem: 56K ITU v.92-ready fax/modem (RJ-11 port)
I bought a 512mb chip a few months ago to bring the comp up to 1gb. All was well. Now I bought 1gb chips PC3200 400MHZ DDR x2, installed them both to upgrade to full 2gb. Darn comp restarts itself over and over. I took out 1 chip to see if defective, same thing happens. Did the same with the other, ditto. So then I looked at the chips. They look the same as the chips i have, fits in same slot, but the label says 1GB 400MHZ 128M. Can someone explain if that means I was sent the wrong chips. I believe I was.