MoBo Lucky Star only recognise 128 Mb instead of 512 Mb Why?

Recommended by Wim's BIOS...

Hallo,
please help me!. You're the only ones who can help me!
I've just added a 512 Mb DIMM Pc133 ram in the last empty slot for ram. In the others two i already had two 128 Mb ram memories. They work fine and they are recognized. But the last one i've added is not recognized, or i should say it's recognized in part. The pc only recognise 128 Mb instead of 512Mb!!!
How could it happen?
Here you are my BIOS details and MoBo ones:

2A69KL1AC-00 Award BIOS number
MOD./Vers. del BIOS: 6ABX2V-G0
motherboard: Lucky Star Apus 6ABX2V/Crus 6AZX2V
ID motherboard 12/21/1999-i440BX-W977-2A69KL1AC-00
BIOS Type: Award
BIOS Date: December 21st 1999
BIOS ID: 12/21/1999-i440BX-W977-2A69KL1AC-00
BIOS OEM: 6ABX2V-G0
Chipset: Intel 440BX/ZX rev 3
SuperIO: Winbond 977EF rev 4 at port 03F0
Manufacturer: Award Modular BIOS v4.51PG

Would a BIOS update work fine? How do i update my BIOS without any mistake?

[cp]

read:
http://www.wimsbios.com/phpBB2/topic3583.html

....so the largest DIMM size supported is 256 Mb. So i could at max reach 768 Mb as i have 3 slot, right?
Thus let's sum up:
i have to search for RAM DIMM 256 Mb PC 133, registered. Modules must be with 16 ram chips, each chip organized as 16Mx8. Is everything right?
Anything else?
Anyway thanks for your precious help......

[KachiWachi]

Nope.

I think you got it.

Thanks.

[Denniss]

You don't need registered/buffered modules.
Ask your dealer for i440BX compatible modules, PC-133 should be preferred as they offer some more headroom for better timings or if you want to overclock via FSB.