What maximal resolution is reasonable when using a VGA cable?

I am not sure there is a limit in the standard.

However:

• Your graphical card needs to be able to provide the image. 400MHz DACs are common, which would commonly limit it to any configuration using a lower than 400MHz bandwidth. E.g. 2048x1536@85 Hz (388 MHz). If your DAC is more capable then you can go higher, e.g. to 7680x4800.
• You monitor needs to be able to recognize the signal.
• With a poor quality cable and high bandwidth signals ghosting is likely. Which can be solved by simply buying a good enough cable.

In your case the GMA 4500 limits you to 2560x1600.

VGA is an analog signaling method, and as such is greatly affected by:

• Cable quality, which includes the gauge (thickness of the wire), shielding, and EM/RF filtering such as ferrite cores
• Output signal from the video card
• Signal processing on the display
• External interference - proximity to power lines, microwaves, blenders, etc.

Most laptops are not well-shielded and don't have clean output to begin with, but in most cases I've seen, standard shielded VGA cables with ferrite cores on both ends yield good results only up to 1280x1024 or 1680x1050 or so over 6 foot / 2 meter lengths. Generally once you start trying 1080p (1920x1080) and higher, you may start to notice ghosting and quality loss.

For a clear image at high resolutions, your best option is to pick a heavy-gauge, well-shielded VGA cable with ferrite cores on both ends.

The best connection I was ever able to achieve with VGA was by using a monitor with BNC connectors (http://www.extron.com/product/product.aspx?id=syvgamalefibncmaleca&subtype=58&s=4), where I was able to get a decent signal at 2048x1536 60 Hz. I'm not sure if you'll find any LCD displays with BNC at any decent price points though - but perhaps worth mentioning anyway :-/

I know there's no standards on VGA connectors/port for max resolution. You should check your laptop's specs to see what the max it will output as a starter. Wikipedia says with sufficient quality cable, up to 2048x1536@85 Hz is possible. A laptop with just a VGA card, most likely the max resolution it will support externally will work fine.