System upgrading i want to upgrade my memory but this motherboard seems to reject most of the available memories in the market.Please advice on the specific memory specification to use and the maximum memory i can insert on each slots on the motherboard.

That's because the video is integrated into the motherboard. This PC supports HDMI... what exactly are you trying to 'upgrade'? If the on-board video is not performing as you want, from what you can see here, you may only need to upgrade RAM on your MB:

http://support.acer.com/acerpanam/desktop/2010/acer/Aspire/AspireM3400/AspireM3400sp2.shtml

#4 at the bottom of the page:
Shared system memory may be allocated to support integrated graphics, depending on system memory size and other factors. Actual system memory available to the operating system will be reduced by the amount of dedicated and shared system memory utilized by the graphics solution and resources required by the operating environment.

667 DDR2 SDRAM DIMMs is fully suited for Intel D102GGC2 motherboard.

Since you're planning to upgrade the RAM of your computer here are the 3 different type of RAM that you can use for this motherboard.

400MHz(PC2-3200)
533MHz(PC2-4200)
667MHz(PC2-5300)

Number of Memory Slots: 2×184pin + 2×240pin
Memory Standard: DDR 400/DDR2 533
Maximum Memory Supported:2GB

PC6400(DDR2 800) is not supported by your motherboard. Also max memory is 2GB. PC4200 is the fastest memory you can install. There might be a BIOS update available from ASUS website to increase the speed but you would have to check the site.

Seems to be a Memory or a Motherboard failure. Make sure the memory you upgraded are the ones which are compatible to the system.
The compatible memory modules for your system are:
DIMM, 128MB, DDR2, 400M, 16X64, 8, 240pins
DIMM, 256MB, DDR2, 400M, 32X64, 8, 240pins
DIMM, 512MB, DDR2, 400M, 64X64, 8, 240pins
DIMM, 1G, DDR2, 400M, 128X64, 8, 240pins

The power light not turning on can be the Front bezel issue and not a system/motherboard failure. Please revert for any more issues.

yes it can be upgrade for 4GB for genuine HP memory. because hp exclusively use hp memory. all Hp pcs uses genuine hp memory.

According to the specs of this motherboard, it only supports 333, 266 and 200 MHz DDR SDRAM. It does not support 400 MHz DDR SDRAM.
This motherboard has only 2 memory slots, each can accommodate a 1 GB memory module. Higher capacity memory modules are not supported by this motherboard.
No BIOS upgrade is available to provide support for 400 MHz DDR SDRAM memory modules.

We actually need to know what your motherboard is. You or let us check if your motherboard can support that memory or not.

Please ask if more help needed

yes you are
take your information to your pc shop and explain what you want to do.
They will then direct you in what type of memory and how much memory your system can take.
( A smaller pc repair shop will give you better advice and a better price then a major shop.)
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You can put new Video card accordingly. Just buy an good Video card which have inbuilt memory and that support your operating system. There are loads of Video card available in the market.