Just wondering does of both of them consume the same amount of memory??... let say i have 5 column in the table.

Although I cant say for sure, but count(*) would in theory take more since it has to find what all the columns are and count them.

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i think they would both use about the same seeing as whichever you use they both just count the nunber of rows.

But the wildcard key gives SQL more to do, SELECT * FROM table is almost always slower than SELECT id,username,othercolumn1,ect FROM table

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i understand that but, and i may be wrong, count is optimized for speed and regardless of the selected column the number of rows will always be the same. so i dont see why this should affect the speed. i might be wrong though.

Saying * still makes it resolve for every row in the table, not a giant difference, but using * will be slower.

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Have a read of this, its very interesting.
http://www.mysqlperformanceblog.com/...t-vs-countcol/

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)