"I don't really follow mobile chips." Me niether Matt, I'm not a laptop expert, but we'll do the best we can.
Oren asked; "Could someone please briefly tell me what are the differences between XP-M 2500+ and XP-M 3000+? Is it much preferrable to buy the 3000+ one?" "Briefly?" No, not if we provide a competent, knowledgable answer.
The 3000 runs at 2.2GHz, the 2500 at 1.83GHz so the 3000 runs 0.17 *billionths* of a second faster, way to fast for the human eye to percieve a difference. No, not prefereable to buy the 3000 and IMHO not worth the difference in price. Both have the 512K Barton L2 internal cache and shine when doing video editing, graphics or gaming.
Oren, when we talk computer modifications several factors come into play and you gave us no specs to go on. Adding RAM is "the poor mans computer turbo charger" because it plays an important role in speed, so if he's running 256MB's it would be prudent to kick it to 512MB's. Find out if the laptop has one or two memory slots because if only one you'll have to buy one stick of 512MB memory and I'd go with PC2700 which runs about a hundred bucks for a desktop. If he has two slots then ask the manufacturer exactly what kind of RAM is in it now so you can match it up because mixing diferent modules not a good idea.
First thing I'd do is contact the laptop manufacturer and ask questions and four come to mind; Will the motherboard support the new CPU, how many memory module slots, what kind of RAM now in it and will the present cooling fan be adequate for the new CPU. You man find answers in the manual or on line so go to the laptop manufacturers website, find the laptop model and take a look see at the specs. Be a sad day if the cooling fan won't be adequate for the new CPU and you stick it in and it burns up . . . .
If you had given us the laptop name and model we could have done the on-line research for you, but, . . we do the best we can with what we have to work with.
Welcome to OC'ers forum, have a nice day and if you have more questions just give a shout. OldBird