INFO
I bought a new HP Pavilioin a610y about a week ago. The type of memory
that came with the computer was PC2100 or at least it was supposted
to. On the box, order forms, hp website, and other websites
(www.crucial.com), They all say this system comes with PC2100.  But
the memory my computer has seems to be PC3200.  The sticker on it
reads --->

hynix           CHINA 53
PC3200U-30330       0431
HYMD264646B8J-D43
512MB DDR 400MHz   CL3.0

There's one stick of 512MB RAM

Also, I went to the setup window as my computer started up and it says
that there is 512MB DDR SDRAM PC2100

QUESTIONs-->

1.  If my system reads the PC3200 as PC2100 will it make it slower
2.  whats the difference between PC2100 and PC3200/ Which is better
3.  I was looking on ebay and one auction said PC3200 was backwards
compatable to PC2100 or something like that; what does that mean?
4.  If I put in more memory, does it need to be PC3200 when added to
the second memory slot?

Hi,

since no researcher is currently working on this topic, i'm going to
answer your questions in the comments.

There are several types of memory.
Recent memory modules are labelled as PC2100, PC2700, PC3200. The
difference between them is the speed the system can access them using
the FSB (Front Side Bus, see Wikipedia for explation).
PC2100 is 266 Mhz
PC2700 is 333 Mhz
PC3200 is 400 Mhz.

Usually both RAM and mainboard are backward compatible, meaning, they
can work with modules fast or slower than for own speed.
For example, your system has a 266Mhz FSB, then it will support up to
266MHz modules. If you put a PC3200 module in it, it will still be
used with 266Mhz, even though it would support 400 Mhz.
Vice versa, if your board supports 333 Mhz and you put a 266 MHz
module in, it will also run at 266Mhz.

Obviously, the fast the memory can be accessed, the better will the
system performance be.

To answer your questions:
1) usually a faster module than support will not noticably slow down
the system. There are exceptions, for example an PC3200 module running
in a 333 Mhz system due to convertions between speeds. But those are
minimal.

2) See above, the faster, the better, but the mainboard has to support the speed. 

3) As mentioned above, faster modules can run on slower speeds (PC3200
memory will be as fast as PC2100 memory on a 266Mhz FSB system).

4) No, it can also be a PC2100 module, but since the price difference
nowadays is almost non-existent, i'd recommand buying another PC3200
module in case you'll ever buy a mainboard which supports the higher
frequency.

Hope that helps.

brgs,
Michael