You're going to be very surprised and possibly quite disappointed by the Atom's performance. I've suffered through 3 generations of EEE PCs and finally threw in the towel and moved a notch upmarket.
Any single-core Atom will perform worse than your 5-year old Pentium M. Heck, in most benchmarks, a lowly Dothan Celeron-M 900mhz will beat the N270/280, and your (clocked comparably to an Atom) will absolutely smack an Atom silly (probably 2x better performance, all other things equal). And the new Pineview CPUs & Pinetrail chipset is just a smidgen better than the ancient 270/280 + 945GSE combo so that's nothing to get excited about.
99.9% of netbooks have 1024x600 LCDsso that's obviously going to be a downgrade. You will pick up LED screen lighting and an integrated webcam, which might be impotant to you.
The Radeon Mobility 9200 is a DX8 card, whereas the GMA950 is technically DX9. That said, aside from Windows Aero, the 9200 will run circles around the GMA950 for DX7/8 gaming if that's your think. Both will of course fail miserably and won't support hardware-assisted Flash for HD streaming etc.
All netbooks now have 1GB DDR2 standard but that's a wash due to the CPU limitation. ANd it's a single stick and netbooks don't support dual-channel. DDR3 is just starting to trickle into the 11.6" ultraportable and ION market.
HDD capacities now are similar (160gb-250gb) though performance, noise & power consumption will be considerably better due several generations of newer portable HDD tech. You will have XP SP3 or Win 7 starter on most netbooks, with a handful coming with Win 7 Home Premium. You can always do a Windows Anytime Upgrade to bump up from Starter to 7 HP.
You'll naturally lose the optical drive. You'll probably pick up another USB 2.0 port or two.
MANY netbooks nowadays still don't come with "N" wireless or Bluetooth. In fact, both (esp. BT) are considered sort of speciality, higher-cost luxury items that you will pay extra for. Most of the cheaper Ralink etc. wireless chipsets used in contemporary netbooks are 2.4Ghz only and don't hold a candle to a solid Intel wireless G card.
Weight & battery life, any netbook or even any current ultraportable will absolutely trounce your trusty HP.
Honestly, your best bet would be something like an 11.6" CULV machine---I am partial to the Acer Aspire 1410 with the SU2300 Celeron dual-core ($400-$450). You also get such goodies as HDMI output and a multitouch touchpad. If you need more graphics horsepower, the Asus EEE PC 120N which has an ION chipset and a dual-core Atom 330 (desktop). It'll give you a nice CPU boost (though far weaker than the ULV CPUs) while offering a HUGE improvment in GPU capabilities and still beat your old HP's battery life.