This is the translation given by the Diccionario Visual de Arquitectura (Francis D.K. Ching). Rosca is defined as: Cualquiera de los anillos concéntricos de fábrica que forman un arco, en especial cuando cada uno de ellos sobresale del inmediatamente inferior.

ORDER
(1) of a doorway or window: series of concentric steps receding towards the opening;
http://www.hants.gov.uk/pevsner/glossary.html

Order
- One of a series of concentric mouldings.
http://www.siteo.net/castleglossary.html

Arco de medio punto is also called Roman arch or round arch.
http://www.thefreedictionary.com/Roman arch

Festoneado - Simon & Schuster's dict. gives festooned or scalloped.

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Note added at 31 mins (2005-05-10 18:16:16 GMT)
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This text explains the \"order\" part:

If the arch needs to be unusually strong, it is better to construct two independent arches, one on the top of the other. Or it may be constructed in three separate rings. Each of these sub-arches, or rings, of which the whole compound arch is composed, is called an order. It is a safer form of arch than the simple arch. ... In some compound orders the faces are in the same plane. But as a rule the orders are successively recessed, i. e. the innermost sub-arch, or order, is narrow, the next above it broader, the next is broader still, and so on.
http://www.catholicliturgy.com/index.cfm/FuseAction/Encyclop...

Picture of a four-order arch here:
http://cloghmore.bravepages.com/clare/killaloe.html


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Note added at 2 hrs 1 min (2005-05-10 19:46:09 GMT)
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After looking at your description again, I think your \"moldura festoneada en la rosca\" is simply referring to an archivolt. Have a look:

ROSCA.- Superficie externa frontal del arco.
http://www.fbarrie.org/fundacion/webcoro/glosario.htm

archivolt - ornamental moldings on the face of an arch
http://www.masterpieces.com/arch3.html

archivolt One of several parallel curved, and often decorated, mouldings on the inside of an arched opening.
http://www.lynnerutter.com/glossary.html

Pictures of archivolts: Fig. 25 on this page:
http://www.sonic.net/~tallen/palmtree/ayyarch/ch5.htm

http://www.pitt.edu/~medart/menuglossary/archivolt.htm

And I think you can definitely use garland.

festoon - A carved or painted ornament in the form of a garland of fruit and flowers tied with ribbons and suspended at both ends in a swag or loop.
http://www.lynnerutter.com/glossary.html

So your phrase: \"Arcos de medio punto decorados con una moldura festoneada en la rosca\"

Could be rendered as \"Roman/round arches decorated with a garlanded molding on the face = Roman/round arches with a garlanded archivolt\" (I\'d go for the 2nd version).