Can any one tell me the difference between iPAQ Compact Flash Expansion Pack Plus and iPAQ PC Card Expansion Pack Plus. Which is better?
In which of the two can I use pcmcia cards tha I use on my laptop?

You can use PCMCIA cards with iPAQ PC Card Expansion Pack Plus.

I recommend this one because you can use it with PCMCIA cards and Flash Cards as well (using a converter).

Are you intersted in buying one?

I am interested.
Will it suite my 5550?
Will i be able to use it with both lan and wlan?

It is compatible with iPAQ h5550. I have one and I was using it with my h5450. This is the combination I have and I consider selling.

iPAQ PC Card Plus Expansion Pack, you will find specifications here
Globe Trotter GSM/GPRS PC Cards (Tri-Band)
Running Voice GSM, you can buy it from handango here