There are benefits to going with the X2 IMO; but with a $300 budget set aside, the real question is what will fall in budget. Now is this $300 just for the CPU, or for the total upgrade?
Anyhow, if you're waiting till January, you might want to see what Intel's announcement might be; and if it will be in time for you to upgrade. (AKA if it's something slated for 4th quarter 2006, you might simply not want to wait, that long.)
An A64-3600+, or if you can still get your mitts on one (as I think AMD is discontinuing them), I got an A64-3500+ using the Venice core a few months back, for about $220. Given the discontinuing, you might be able to find one for "bargain basement" prices, though you might need to do a little digging on online shops.
Personally, I went with an NF-3 chipset, rather then Via... And needing AGP is just going to be needed if you're to stay in the budget you slated.
Now some good, and some bad news.
The good news is that once this upgrade is done, swapping out for a mobo that supports PCI express won't be that expensive and can be coupled with a gfx card upgrade. The mobo I got was about $82. Against the cost of a new gfx card, eh, that's penuts by comparison.
If you decide to go X2 latter on, the mobo should support it, making it possible to get a BIOS upgrade (if needed), and pop one in. Course I'm not sure what BIOS your mobo would ship with.
Now some possible bad news:
- The socket 939 uses a 128-bit memory bus, and as such to make full use of this you'll need paired DIMMs. You probably could get away with using a single DIMM (though I'd have to look into this); but then not be able to use dual channel memory mode.
If you're DIMMs are of mis-matched types for your Pentium 4; then definitely get a Venice core, as among other improvements were greater compatibility with the on-die memory controller, and support for mis-matched DIMMs added. New RAM, might or might not be necessary if your DIMMs aren't paired already, though you can always try it.
- You might or might not need a new PSU. Again it depends what you have. Recomendations I've seen is that you want at least 18a on the 12v rail. My old 400 watt PSU was below that with 13a; so I replaced...