The X1600 is faster than the GF7400, but if you're interested in gaming you should look for a GFGO7600 or even GFGO6800 which would outperform both of those cards you listed.
Out of your two selections the X1600 is faster, and the one game it would be worth getting for instead of the other two options I mentioned would be Oblivion.
Now the other questions is what else are you going to use it for because the performance in certain areas swings a bit.
ALSO, be advise, without changing the driver settings the X1600 will throttle back alot when running off battery and will likely underperform the GF7400, so if you plan on gaming on battery and don't mind that being for like 60mins then change the driver settings to always run @ full speed.
Would it be faster than my computer that I have right now? I have an Acer Ferrari 3400LMi:
Mobile AMD64 3000+, 2,0Ghz, 512Kb
1,25Gb DDR333
ATi Mobility Radeon 9700 128Mb
The GF7400 with that combo would be a little faster MR9700 is really just an overclocked R9600), the MRX1600 would probably be much faster. My MRX700 is faster than the MR9700, and the MRX1600 is faster than my MRX700.
Both would offer more features though (both for gaming and movie watching/editing, and would also consume less power (PCIe is great for that).
Look at the Tom's review and you see it beating the MRX700 in the Acer Ferrari by an average of 40% in games, and that graphics chip like I said is better than your current one.
Well it's supposed to hit retail the week I fly out to Toronto, so hopefully there'll be some early models there to look at (hopefully at least an MRX1650 too [but that's unlikely as it is just going to hit Desktop then]).
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