First a short explanation what your miner does: it calculates the (double) sha256 hash of some data. The resulting hash has to have some leading zeros. The number of leading zeros is the "difficulty": The more zeros, the more difficult it is to find it. A miner changes some bytes of the data it hashes, to generate different hashes.

Most pools support different protocols with different options. The original protocol was getwork. But this allows only 2^32=4.29 GH for one request. For fast ASIC miners with 600 GH/s, the limiting factor would be the network speed, not the mining speed, because the miner was allowed to change 4 bytes only. There are some extensions to increase this limit, but it is not a nice an extend-able protocol. This was the reason to invent getblocktemplate. It allows to change much more data on the miner client side, and the miner decides what transactions are included in a block.

If your mining software gets a "share", it mines it until it finds a hash for the requested difficulty. Then it sends it to the server. There are pools which adjusts the difficulty, depending on how fast your client solves a share. The client can actively request new shares, or it can use the longpoll option, which means the server sends new shares, if it thinks it is necessary (e.g. there are new and profitable transactions with fees which can be included in a new block and which might require a new getblocktemplate). Easyminer supports getblocktemplate, so it is not important how many shares you get.

The profit calculator you cited is for using the miner with a pool. Except the solo mining days number: 148 days would be the average time you need to solve a block on your own. the difference to pool mining is, that you create your own block (this is similar to getblocktemplate), but the difficulty is not from the mining pool, but it is the current difficulty from the bitcoin network. This makes it much harder to find some data, so that the resulting hash satisfies the difficulty. But if you mine a block, you'll get 25 bitcoins for it. If you are lucky, you could find a block after a week, but if you are unlucky it might need a year.

The required processing power for your computer for a PCI card is very low. The computer is only used to get the work from a pool (or the bitcoin network for solo mining) and then waits for the card to brute force a lot of hashes. But you need a good power supply and cooling system.

The problem with all the current mining hardware is the availability. You can pre-order it, but when you get it (sometimes you have to wait months for new products), the difficulty of the bitcoin network might have increased already, so that it needs longer until the hardware break even. Or the exchange rate for bitcoins to a fiat currency dropped, if you want to exchange it.