Processing popwer these days is vast, compared to PCs of only a few years ago. What you have asked for will be done admirably by a cheapy, but FS is going to make it sweat just as much as anything, including fast arcade/shoot 'em ups, etc. Thing is with FS that you ideally want at least 512mb RAM and preferably 1GB, plus a seperate (64MB at least) video card. This is no real sweat and your chosen supplier will often do an upgrade for peanuts. Extra memory will give much more elbow room and give the impression of a faster and smoother (for longer!) system
The screen? LCD is superb for living with and for general purpose useage, Once you have one, you will need a good reason to go back to CRT. Even with an LCD flat screen, don't go for less than a 17 inch screen, which is rapidly becoming the default size.
Hard disk? Even 80GB is a pain nowadays, but quite adequate. Another ten quid will probably get you 160GB though!
Other features come into the odds and sods category and can be added later if needed.
The only caveat to the above, is this. If you are going to splash out, get the most powerful machine that you can reasonably afford, as it will be so much longer before you are back in the position that you are in now. A cheap buy now, will ideally still be seen as being good value for money in two or three years.
Let us know if we can help,
Conan