I have 3 NICs, one that connects to my internal network, another two that connect through different ISPs to the internet. The NICs connected to the net are both the same, as in RealTek 8139(A).

One of the connections if extremely stable, max throughput is 30K/s inside my country and 15-30 outside. The other is very fast inside my country (up to 5 megs/sec on ftps), but can get a real pain in the a55 on some outside connections (2-12 k/sec) while on some other it can go as high as 200-300k.

My ideea was to somehow route the packages through the convenient gateway, as my LAN connection requires it. What I want to know, before the "howt o?" is what do i need in terms of software (kernel patches/a.s.o)?

You just need to use the "route add" command. See:
man route

Hmmh... I don't think it's that easy as adding a "simple" route.. I read something about packet marking or stuff like this... in which case... again... is here any software I'd require?

On what basis do you want to route the packets if it is not by IP/subnet?

<fx> Hijacks thread </fx>
I have a machine with 2 network connections (eth1, 192.168.1.251, and eht0, 10.0.0.2) the default gateway is 192.168.1.254, how would I add a route for specific services, for instance all traffic from my FTP and web servers go throuth eth0 (the internet gateway on that network is 10.0.0.1) but everything else go out through eth1

Thanks, my flat mate moved out so I have 2 ADSL connections for a while, his is a bit slow so I'm using it for big downloads but want the same computer to be FTP and web servers on my connection.