An overclocked Opteron 165 or X2 3800 will outperform an overclocked Pentium D. It'll also be cooler and use less power, meaning it costs less to run. So the extra outlay will pay for itself in time, and that's before you even consider the cost of a new motherboard and new RAM.
My suggestion if you want more power now is to stick with AMD. Buy an X2 3800 for less than £200 and get back £100 or whatever from selling your 146. Leaves you with a £50-70 upgrade to dual core and a lot less hassle. Sure, Conroe will be faster, but the likelihood is K8L will be faster than that, and Core 3 will be faster than that, and then there'll be K10 (whatever happened to K9 anyway?!) and then... you get my point.
Buy for today. If you're going on holiday, save the money. Conroe will cost you several hundred. Chips will probably have parity with current Intel high end parts, a decent new Conroe compatible motherboard will be the best part of £100. So the reality is that you are looking at £100 or so for dual core AMD, perhaps as much as £400 for Conroe.
Get an X2, enjoy the extra power, forget that some people will have computers that will bench higher than yours since yours will still do everything you need it to and is still really powerful and enjoy your holiday.