PLIM and WIC-1ADSL

Hi all, I've got this problem: my company has an ADSL line 2048/512 kbps, but we can only use a bandwidth of 500/600 kbps. After a lot of checking with our provider I saw that:

sh atm interface atm0/0 Interface ATM0/0: AAL enabled: AAL5 AAL2, Maximum VCs: 23, Current VCCs: 1

Maximum Transmit Channels: 0 VCIs per VPI: 256, Max. Datagram Size: 4528 PLIM Type: ADSL - 640Kbps Upstream, DMT, TX clocking: LINE

655811 input, 628328 output, 5907672 IN fast, 5353092 OUT fast Avail bw = 640 Config. is ACTIVE

The available bandwith is 640 kbps because the PLIM type is ADSL -

640Kbps.

My router is a Cisco 2620XM and the interface is a WIC-1ADSL ... I don't see any bandiwidth limit for this card, so why is it so low ?

We have also Cisco 827-4v and 1721 ADSL, for these devices the Available bandwidth is 800 kbps and ... 320 kbps!

What can I do to have an higher available bandwidth ?

If it's a card problem, how can I choose the right one ?

thank you to all!