I guess you will have to decide whether to run XP or Vista. XP will be cheaper to buy (unless you get either for free... somehow...) and uses less RAM so you have more to spare to run ETW (min specs for XP and 1GB RAM but 2GB for Vista). On the other hand we don't yet know how well optimised the game will be for different gfx cards with XP and Vista. Possibly Vista will run it much better for a given card due to the optimisation that might really on some Vista trick that XP doesn't support. This may mean that what you save on the gfx card makes up for the money you spend on RAM.

It's all speculation though at this point of course. As other have said, maybe your best bet will be to go for a fast dual-core CPU, a mobo that can handle it, 2-3GB of RAM then the best gfx card you can buy with the money left over in your budget.