The Barton Core is the latest version of the AMD XP. And the initial ones i have seen reviewed are rated as equivalent to 3GHZ (XP3000+). The main difference is they feature an extra 256k of L2 cache on the chip, which in theory means the Chip will be able to prefetch twice as much into extremely fast CPU memory than before.
However from the reviews, it would seem that it hasn't added lots over the thoroughbred XP2800+ and i have seen pre-order prices for the XP3000+ @ £500
If i were you, i would buy either an XP2100+ or an XP2400+ and buy some faster ram and moderately overclock up to at least 166FSb (You should get a XP2400 to run at XP2700 speeds easily with a decent heatsink and PC3200 Ram)
This with a Radeon 9700pro should handle nicely the current games with high detail, though there will always be a game round the corner (Doom 3) thats going to push things to the next level
Overclocking is dangerous to CPU health if you overdo it (Mainly Voltage for RAM and CPU), but my current machine has been running overclocked for 18 months and absolutely no problem.
The main problems come if you are really trying to push the components for high scores on benchmarsks, but overclocking moderately not only saves you money, but these days is pretty easy and supported directly by most enthusiast motherboards
There are numerous forums where you can read all about this (A place to start is
here ) they sell all the kit and have a good forum
IMHO would upgrade the following:
Motherboard - Abit NF7-S (wait for revision 1.2)
CPU - XP2100 or XP2400
RAM - 512 (2x256) Corsair/OCZ DDR Ram (Used in dual channel mode)
GFX Card - Crucial Radeon 9700Pro
The abit board has decent sound support and a digital out connection, so you may not need your Sound card