I guess the question that immediately presents itself is: why do you believe you want/need to upgrade the memory later?
While Dell is probably considered a premier brand, don’t underestimate Acer. Good quality, and usually very good value for money. The D610 looks like a decent bit of kit, but only has a Pentium M processor. Realistically, I don’t think you’d ever want to upgrade it beyond 2GB – and remember that Windows XP (which is what I imagine the D610 runs) can’t make use of more than 3.5GB anyway. XP will typically run very well in 2GB or less (even with 512MB it would be fine). £89 seems a very good price for a machine of the D610’s spec. That said, it was launched 5 or so years ago. There’s a positive review here:
It looks like it cost £1,000 at launch!
Hard to say whether it’s worth going for the Acer without knowing more about its spec. If it’s got 4GB of memory installed, I’d guess it would be a much more recent machine, probably with Vista or W7. I’d expect to pay much more for it.
In summary: if you just want a laptop for Internet, basic office functions etc., I’d have thought the D610 was a very good buy. I’d be interested to know what warranty you’d get from CC: I’d guess 3 months, which isn’t so bad.