Try to specify what kind of you computer you need right now is impossible because models are always changing. These are typical components used in PCs in mid-April. They range anywhere from about 450 Mhz up to 1 gigahertz and more. Windows 98 is intended for everyday consumer or home office use, while 2000 is intended business users who need more stability or intend to run networks. A system may come with between 128 and 512 Megs.
More memory means the computer can keep more software and data running at one.
The low end is fine for many applications, though it may run a bit slower. These vary depending on vendor and technology. Typically, any card will work fine in business applications, unless one uses high-end multimedia applications. A 56K unit is standard everywhere. Until a new technology comes out that crams more data into a phone line, these will still be plentiful.