Just bunged in a stick of 512Mb DDR to double my memory but PC is slower?? Help!

Not still using Win98 are you? Anything over 512MB can cause problems.
If not, then the only other thing I can think of is that your RAM sticks may not be compatible with each other.

Something sounds wrong (duh). I doubt you'd notice any speed increase, as 512 is more than enough. But a slowdown? Are you using a matched set? Have you tried your new stick by itself, in the same slot as your original, to see if it behaves speedwise like your original? It could be something as simple as a defective stick. Can you borrow some known to good memory to try? Are you an Intel guy? Some AMD boards don't work right if all three slots are filled. That said have you checked any FAQ's at your mobo's site. Maybe a BIOS update is in order.

For optimum performance one large stick will always be the fastest (especially if you want to run CAS 2 timings). But this is more of a benchmark reading than the actual 'feel' of speed.

What OS ?

Runnin XP on a 2000xp chip existing stick of 512 worx ok, other stick worx ok in another machine. Three slots so with this xtra stick only 2 are filled [oo err missus].

Will take out the new stick and report back.

Hi JW

Just checking - some older OS's had limits.

As has been said - are they matched ?

Well bub,

MATCHED PAIRS u r pullin my dongle, they are of course the cheapest available.

Just wipped the new 1 out and if anything things are worse