Either version is fine but make sure you flash using the factory FW first then once you're on openwrt already then you upgrade with the sysupgrade FW. So you need to download two FW of that version you going to use. I would recommend these two:

openwrt-ar71xx-tl-wr841nd-v5-squashfs-factory.bin  20-Nov-2010 04:30            3932160
openwrt-ar71xx-tl-wr841nd-v5-squashfs-sysupgrad..> 20-Nov-2010 04:30            3547140

Yes, maxis uses VLAN 621. 500 and 600 are UniFi's for data and IPTV respectively.

Last week just tested Asus RT-N56U with Maxis Business Pacakge.
Just give some sharing as below:

Maxis Business Package: 4MB
Model: Asus RT-N56U

1. Going Asus Router Interface and choose WAN Setting.
2. Under "Connection Type" Choose PPPOE
[attachmentid=3010760]
3. After find "Select ISP" Choose Manual
4. In the "Internet" Colums Key in VID 621 (Maxis Internet VLAN Digit)
[attachmentid=3010768]
5. Apply it.
[attachmentid=3010769]

No need flash any custom firmware for Asus RT-N56U!!!

Hhmm. Guys. I'd like to confirm 1 thing on this.

1) Maxis uses 2 diff Vlan ID for HSI
      Those using TM Modem (Vlan Tag is 621)
      Those using Modem provided by Maxis (Vlan Tag is 11) [The following setting is based on this]
Please refer to Attachment. This is a Thomson Config file i've downloaded for backup.
[attachmentid=3007675]

Also I'd like to share my experience using TPLink 1043ND router with Huawei Echolife HG8240 & Thomson (mainly for VoIP)

All Vlan Tagging are done on the TPlink router hence there is no need to configure the Echolife HG8240. *I'd like to point out that not everyone is done in this manner, hence please check before following the guide.

This is how my wiring setup
[attachmentid=3007694]
This is how i've configure the Switch & Vlan Tagging Setting (Under Network --> Switch)
[attachmentid=3007695]
I also like to highlight that Port 1 in the image is WAN port, Port 2 is Eth 1 ... Port 5 is Eth 4
For my case i used Eth 4 hence is port 5. Take note on the Vlan ID 1 under Port 5 change it to off. Add Vlan ID 11 and change Port 5 to "Tagged" & CPU to "Tagged"

then save & apply

This is the WAN Setting (Under Network -- > Interface --> WAN)
[attachmentid=3007697]
Change the setting to VLAN Interface eth0.11

then save & apply

Then under General Setup  (Under Network -- > Interface --> WAN)
Change protocol to PPPOE
enter username xxxxx@home.maxis.com.my or xxxxx@public.maxis.com.my
password is usually the xxxxx1 (where xxxxx represents the 5 digit before @home.maxis)

Save & apply..
You are done