I'm buying a laptop from Dell(http://www.dell.com/content/topics/segtopic.aspx//inspiron-14r?c=us&cs=19&l=en&s=dhs&~oid=us~en~29~inspiron-14r-anav3~~&redirect=1, scroll down to see $849.99 option), and it comes with an ATI Mobility Radeon HD5470 1GB. How does this GPU match up to desktop models with similar specs? Are there any differences? And is this good enough to run modern games on decent settings (I've been told it is, but better safe than sorry)?
The short answer:
The closest desktop GPU card that it performs to is a Radeon HD 3670 or a Geforce 315. Look up how these cards perform and you should expect similar results.
My suggestion would be getting a Mobility Radeon HD 5730 for decent settings or a Mobility Radeon HD 5850 that costs a bit more (comparable to an 8800GT).
Long answer: According to passmark benchmarks the Mobility Radeon HD 5470 has a video benchmark score of 308.
A decent desktop card to run modern games that you could have medium-high settings like an 8800 GT has a video benchmark score of 945. Or the Radeon equivalent which is a Radeon HD 3870 has a score of 961.
Notice that there are more powerful mobility cards like the Mobility Radeon HD 5850 that could perform for some really decent graphics.
You can check it out yourself here: http://www.videocardbenchmark.net/high_end_gpus.html
I believe I have a Dell similar to yours and I'm going to tell you that it's not that great for gaming. You would have to run most of the settings on low or low-medium.