Memory for my new motherboard

Hi,
1) Can someone please explain what this notation means (related to memory, ddram I think): "2.5-3-3".
I read somewhere that 2.5 is the "clock cycle time", what about the other two?
Given one motherboard/chipset, is it possible that it works with both cl2.5 and cl3 memory (I mean not at the same time). If it is,

then one should always buy 2.5 cl if he wants to achieve the best performance?

2) My board is an Asus P5GDC-V, based on Intel 915G, I'm looking to buy 2x512MB DDRAM 400 for it. It has 4 DDRAM 400 slots, with

dual channel. There is a list in the user's guide with the "qualified" memory models. For each model it is specified whether it

works or not as 1 DIMM - single channel, 2 DIMMs - dual channel or 4 DIMMs dual channel configurations. Some don't work in 2 dimm

and quite a few are not useable in 4 dimm configuration. What factors determine this compatibility?

I now have this dillema:

In mb's qualified list there is this Apacer memory model:
512MB Apacer 77.10736.19G, based on "component" Infineon HYB25D256807BT-5B (double-sided), which model supports all three

configurations (1, 2, 4 dimms).

In a local store I found this:
512MB Apacer 77.10736.33G based on "component" Infineon HYB25D256800CE-5
This particular Apacer model does not appear in the qualified table.

However, I found this qualified model:
512MB Infineon HYS64D64320HU-5-C based on Infineon HYB25D256800CE-5C (note the "C" at the end)
This also works in all 3 configs.

Now, is it safe to assume that since the Apacer model in the store and the Infineon model are based on the same chip (I hope the

"C" is irrelevant), the Apacer also supports all 3 configurations? Also the model name looks quite a lot like the first one I

talked about, which also supports everything.

Also, all are CL3, should I forget them and go for a 2.5? I'm not sure if any 2.5 is in the "qualified" list, didn't look too much,

is it possible that there are none?

3) What kind of packaging does memory usually come in? Are there "boxed" and "oem" versions like for processors? I know these guys

just give you the module naked. Is it advisable to buy from them?

Thank you,
Lucian