Thank you all very much for your prompt, detailed and helpful answers!!!
Your questions are very sensible. I'm placing a custom order; the budget really needs a starting point from what I need to run sl comfortably and then if necessary go down from there. Windows, yes. Monitor, will more likely need a new one. Keyboard and mouse can be reused. I'm in Latin America and yes, planning to buy as soon as possible. Thanks for the advice on graphics cards prices speculation; I don't think in my situation it will make much of a difference however, as they are imported and if prices go down, the local currency-usd dollar exchange rate goes up anyway so my guess is that will even things out.
This is just what I need! Thanks so much for posting all the details! I love taking photos, I'm usually on a tiny community sim which (unless there's a big event) are normally not very laggy and usually run only one avatar at a time now that I'm not rping but could use a second one for some photo shoots. I'd be super happy with high and if I can get a bit higher even better. Your post really gives me the starting point I need. And everyone else's advice helps me adjust things if my budget doesn't fit this or if I can extend it a bit, for example to 8 or 16Gb.
I'll have a look at all the articles you suggest Lindal, Lilith and Nalates (I will pass on your info and advice to my technician as it's really too technical for me to understand but he'll understand it and it's very valuable).
Once again, thank you all very, very much for such helpful answers!!!