More RAM never hurts any system. However, never buy RAM from Apple. Their prices are ridiculously high. Shop around and you can find RAM for generally 1/4 of Apple's price.
You really won't notice a huge difference between 2.4 and 2.5 ghz. Thata's like, what... 0.0000001 second in time?
When I purchase my Macs I get the base configuration from Apple then purchase RAM and hard drives from other vendors that aren't hell bent on raising prices so much. Check out www.otherworldcomputing.com and www.dealram.com for RAM proces and hard drives.
scott's right, dont get RAM from apple. download the "crucial mac scanner" it will get the necessary info from your computer and will tell you which memory to buy. i always just buy directly from crucial. their prices are reasonable.
i have an older G5 which only recognizes 3 gigs of RAM and i havent had any problem running photoshop/ illustrator at the same time.
Hi, I'm running one of the new i MacBook's (i7) with it's 'off the shelf' 4gig Ram. Running CS5 PhotoShop which is the one which will pull on it (Illustrator shouldn't given it's mainly vector... unless you're doing lot's of fancy effects which one should really think about doing in PhotoShop anyway). I've been messing about with PhotoShop files up around the 600mb mark and it's eaten them. Get higher.. 800mb and you start to feel it a bit, but it's still a reasonable fast screen redraw. These are fairly heavily layered/masked jobs with a reasonable resolution for large 3m banner work. The next level up is what I'd class as really high-end specialist photo work... which you'll know whether you're in that field or not. Hope that gives you a feel. All the best.