I bought an Acer 5315 a couple of years back to drive the mount and cameras and got it for about $400 after factory cash back which was pretty cheap at the time, but it was a 2gig celeron with 512meg ram with vista installed. Naturally it was a pig, so I upped the ram to 2gig, max allowable according to acer, and initially installed XP. It ran just fine for a $400 laptop and as I did image processing indoors on the quad core, photoshop responce was irrelevent.
I put Win7 on it a while back and it has started showing it's age lately so while I was searching and messing about with xp mode a week or so back I came across an upgrade thread
here. That prompted me to go looking for a new cpu to suit. Of course they are off the market but I found a 2nd hand chip on ebay and have installed it today.
That chip drags the old laptop from a single core 2gig celeron @ 533 FSB to a 2gig Core 2 Duo @ 800 FSB and the difference is remarkable. I'll get another 2 years out of the old girl yet and for under $100
That upgrade page notes that some people have installed more than 2gig of ram and it works fine. Round 2 coming up
Maybe you can find a page in there for your laptop...