I'm looking for a broker that can handle 50-100 trades per day
(daytrading).

There are two important criteria:

- execution of the transactions. I want to trade a portfolio of 25 -
50 stocks, buying in the morning, closing in the evening.  The list of
stocks and the number of shares is known in advance. The broker must
be able to execute the trades first thing after opening and just
before closing.

- Price. I'm looking for the cheapes solution possible. (fixed fees,
or percentage of volume traded)

I'm interested in brokers that can handle the following stock
exchanges:

- NYSE
- NASDAQ
- XETRA (Germany)
- Virt-X 
- SWX
- IBEX

(or a subset thereof)

You might be better served by looking for an answer to this question
someplace like "elitetrader" where there's a good community of very
active traders that freely offer their opinions on brokers that cater
to high volume traders with special needs.

here's the URL to the broker review/opinions section of elite trader
http://www.elitetrader.com/br/

Thanks for the link - that was really helpful and pointed me to <a
href="http://www.interactivebrokers.com">InteractiveBrokers.com</a>.
They seem to fit the bill...

Please be careful.  Daytrading is very high risk.  Make sure you can
afford to lose the money. Secure your financial future first, then
play with some of the extra money.