Crucial isn't the cheapest but you're guaranteed it'll work (or they'll refund you money).
Having had a look at the Crucial recommendation for your system click here, it would appear that it'll take single-sided RAM only - Slots: 4 (2 banks of 2). But I could be wrong.
If it'll take double-sided RAM, then Play could be amongst the cheapest click here (buy DDR2 desktop memory). Or, if you're an eBay member, you could pick up a bargain - just make sure to buy the correct type: DDR2, 240-pin unbuffered & non-ECC. G
I agree crucial has a great tool, but their prices are quite a bit higher.
in the end i went with dabs who seemed to have the best price. its only value ram, but that should be ok.
on another note, its intresting how crucial scanned my system and only offered me an extra 2gb of pc2-5200 (to match what i already have). i guess they are assuming you want to add to your current ram and keep the same bus speed, whereas im planning to upgrade the bus speed too and replace the whole lot
"...whereas im planning to upgrade the bus speed too and replace the whole lot"
You'll notice next to no change by going from 667Mhz (PC5300) to 800Mhz (PC6400). I would save the money, or invest in something else instead - like a dedicated graphics card for example, if you have onboard at the moment. Nothing too fancy or expensive - a £20-25 one should do nicely over onboard graphics.
I'd be interested to know if the RAM you purchased from Dabs is single or double-sided, & if it worked alongside the RAM you have now. So please do get back after you've installed them.
I've purchased RAM several times from all over (including on eBay & secondhand) & never had a failure or problem with any of them. G